I would like to plant some shrubs and/or flowers in a mostly wooded area behind my house (in NE Oklahoma). What are some good candidates that are (1) cheap, (2) perennial, and (3) hardy enough to survive without any assistance from me? (This is just for fun -- nothing serious, so I don't really mind if 3/4 of them don't survive.) Invasive species are fine, poison ivy is not.
I don't know of a list of plants for Oklahoma, but I'll tell you how we achieved the criteria you listed for flowers.
Drive around in the spring (we're in a rural area), notice what is flowering in the ditches on the side of the road. Dig up some of those plants, bring them home and plant them. Here, some of the early blooms are from the lupines. (Obviously you don't want to steal plants from someone's front yard, but digging up a scoop of dirt and weeds from a roadside ditch won't bother anyone.) Repeat in mid- and late-summer and you'll have something flowering all summer long. Not everything you find will be perennial, but since they are weeds, they are likely to reseed freely. Since you're getting plants that are naturalized to your area, and are already growing without any human help, you're sure that they'll survive without your help.
Given that you have a mostly wooded area, I presume it is somewhat shaded. So when you're scouting wildflowers, keep an eye out for what's growing in the shade. You should also keep an eye on where they're growing and what kind of conditions you're going to put them in -- weeds growing in a ditch that's always wet may not grow well on sandy, very well drained soil. Of course, even if you fail on the first attempt, you'll learn about what will and won't survive in your planting area, and anyway, you said you don't care if you have a high failure rate.
Shrubs are a little trickier, but you can use a similar strategy. Find some nearby family, friends, or neighbors with vigorous (possibly overgrown) shrubs. Get permission to take cuttings, root them, and then plant them. Before you take cuttings, ask the owner about what kind of care they have to provide for the shrub -- if it's something that requires more care than you want to provide, you may want to pass.