I think Kuat makes the best bike rack accessories, but "affordable" might not be the best description. I guess when you compare to similar items from Thule or Yakima, the prices aren't too dissimilar. I'd pick RockyMounts as a #2.
You probably already know most of this, but I'll write for others who are looking, based on my experience with the MkIV and Mk7.
(1) Trunk Rack: Cheapest and "easiest," has some drawbacks.
"Easy" on and off, and the cheapest solution, but you may lose the ability to easily access items in the trunk, the bikes may be less stable, and you may have to mount-dismount the rack and store it for every use. Some trunk racks are not happy with odd frame shapes/sizes, fenders, rear-racks, or other accessories, and some will hold your bikes with crankset against disc brake against spokes. You'll see little loss of FE, if any, as well as see little out of your rear window, and perhaps you'll get some wind whistle. Nylon web straps will need checking, especially if they see a lot of use, and don't overdo any corners or rough roads. If you leave the rack mounted in an unsavory area, be aware that most can be removed and taken. If you pull over for errands or food and walk away, bikes secured to a trunk rack are the most likely to walk away, too.
I'd suggest a Saris Bones over Thule or Yakima. The Bones-3 is +/-$150, the Kuat Highline-2 is $190.
(2) Roof Rack: Costs more, has some drawbacks.
Cross bars will allow you to carry other items on the roof, but otherwise you'll need additional roof racks to carry bikes- and be ready to deal with lifting overhead (and debris going from crankset/cassette to roof), along with wind noise, and loss of FE. Watch out for low clearance, high winds, heavy bikes, and the price of adding bars and racks. Once the cross bars are on, are you leaving them on, or taking off seasonally? Storing the roof racks? Using the bars for other sports? Once the bikes are on, you'll get wind noise and a definite drop in FE. Some roof racks will not play well with fat tires, fenders, tires that lose air, or odd frames shapes, etc. Good cross bars require a few minutes of thief-work to remove, and good roof racks will have some method of securing the bike to the rack to the car. A cable lock on the roof is only as good as a cable lock on the trunk, so look for one of the better systems if that's a concern to you. IMO, cross bars and accessories are best if you are using them for multiple sports and hauling stuff.
Whispbar made the best cross bars and roof racks, but after Yakima bought Whispbar, the cost of the old original items seem to have gone through the roof (if you can even find the right ones).
I'd pick used Whispbar cross bars and T-Slot accessories- (this makes life much, much easier), the Whispbars are silent, and no hit to FE once bikes are removed, even when you leave bike racks on. Yakima and Thule both have bars that now copy the Whispbar design, if they have T-slots they might be worth a look, but will be pricey. For roof racks, I'd look at ones that aren't bothered by frame shape (holding the bike by the tire, or down tube) rather than by the QR dropout or TA fork, but if you have fenders or racks, this may not work. These are, of course, often the most expensive.
(3) Hitch Rack: Usually costs the most, has some drawbacks.
If you don't have a hitch, and don't tow, then this is usually the most expensive option. If you already have a hitch or tow things, it's a different ballgame altogether. The cost of a hitch will depend on the brand and install price, if not DIY. There are lots of posts on this topic...Curt/Draw-Tite/Ecohitch/Bosal-Westfalia, from $150-$350+. Note that if the installed hitch is too low to the ground, or your car is lowered, you may get ground contact in some circumstances.
Hitch racks come in two main types: platform/tray and hanging/suspension. Each comes in different varieties, from swing-away, to tilt, to leveled ("levels, Jerry, levels"), to single/double arm....etc etc. The platforms are more stable, but more costly. The suspension racks will still require lifting, but not as much as a roof rack. There is less wind noise than roof racks, more stability than most trunk racks, and little FE penalty. The hitch rack is usually less prone to theft, since most are lockable to the receiver (and more clumsy to steal), and good ones make theft of the bikes more difficult, too. The view out the rear window may be obscured, and some hitch racks will interfere with accessing the trunk. Platform racks are bulkier, can be clumsy to insert into the receiver, and need more room for storage. Most will fold upwards when not in use, which is nice, but occasionally LE will complain they can't see the car's rear plates (this can be a complaint about trunk racks, too). Hanging racks are usually lighter, easier to mount in the receiver, and need less space for storage, but some won't play well with odd frames/small bikes/accessories, etc. Some platform racks don't work well with fenders, fat tires, etc.
I think the Kuat Sherpa is the best hitch rack for combination of quality, ease of use, and cost, but I prefer platform racks to hanging. I'd look for a good used one (original version or version 2.0) on Craigslist and pounce on a good deal. If you prefer a hanging rack, I'd pick the Kuat Beta, (which may have a replacement soon, and since I haven't checked it out, would not venture to offer a rec). RockyMounts has some great hitch racks, but they are not cheap.
Last word: I have friends that are very content with their inexpensive Allen/Hollywood/Chinese racks. They have them for about three years and buy new ones and are content again. And then they buy new ones about three years after that...
