The cheapest option is to go with a foam pad under you instead of an underquilt. A popular option is the blue foam pad from walmart. They are only around $10 I think. If you can, look for the 26 inch wide one that is sort of a "waffle"-type raised pattern. I had to look in four different walmarts before I found that type. The other type they carry is only 20 inches wide. It will work, but your shoulders might get a little cold. You could also get a thin foam pad from Gossamer gear. http://gossamergear.com/sleeping/1-4-wide.html
These also work well.
Many people stick with pads and are very content. They can take you down to some pretty cold temperatures, as long as you have a decent sleeping bag or quilt on top. A lot of people feel that it is hard to stay on a pad and that they aren't as comfortable. They eventually move to underquilts for comfort. You can read the bottom insulation thread for opinions about down v. synthetic underquilts. There are several cottage manufacturers on this site that make good underquilts. To stay inexpensive, I went with a 3/4 length synthetic Jarbridge River underquilt from Paul at Arrowhead Equipment. I got it on one of his sales. I think the regular price is about $125. I love mine. If you get a 3/4 quilt, you will need a small pad to go under your feet. I thought Wilderness Logics made an inexpensive underquilt, but I don't see it on their site. Hope this helps.