Before all the self inflating pads hit the market (sometime in the late '70s/early '80s), everybody used closed cell foam pads (AKA cheap blue foam). Cheap, lightweight, kept you insulated from the ground but they were pretty bulky. You can still get them at Wal-Mart, REI, and possibly a few other places. If you're camping in mild weather (which most cycle tourists are), on a budget, and don't mind a little bulk, get one from Wal-Mart or REI. If you want more padding between you and the ground, get two. They're cheap enough that you can cut it to the length you want if you're a short person, or cut one up to use as padding and insulation for stuff that you want to protect. It's also great for car camping. http://www.rei.com/product/374059

Therma-Rest offers a folding foam pad called the Z-Rest. Still cheaper and lighter than a self inflating pad, but easier to pack and a little more compact than the cheap blue foam pads. It folds up like an accordion, so it doesn't try to unroll itself like the cheap blue foam. I use one when I'm backpacking instead of a Therma-Rest. I can use it as a knee pad when it's folded up, for stuff like getting my stove going or packing my pack, and I unfold it partially to sit on. It's a little bulky, but I just tie it to the outside of my pack. http://www.rei.com/product/781089

The Therma-Rest Pro Lite is my choice for a self-inflating pad. It's as lightweight and compact as a self-inflating pad is going to get, and if I inflate it a little by mouth after I unroll it it does an excellent job of insulating me from the ground. It's great for a tour where you have to fly to or from the start or finish of the tour, because it rolls up into such a compact package. I use the stuff sack that Therma-Rest sells, it's exactly the right size to roll up the pad into the smallest package possible. If you combine a pad like the Pro Lite with a really compact summer weight down sleeping bag, you have a really compact package that can fit inside a pannier instead of having to be tied to the rack of your bike. http://www.rei.com/product/780988