I think part of it will depend on the size of prefold, but since your child is getting up there in size, it probably wouldn't be too bad. When my DS was 3 months old and 7 lbs (preemie), he was peeing out of an infant prefold, but the regular size was WAY WAY WAY too big for him. Now at toddler age, he can wear a regular size prefold no problem (and it holds plenty), and it's quite trim. I use the bikini twist. It's much trimmer for him than a newspaper fold was (I fold excess down in the front, of course, since he's a boy... folding down in the back might make it trimmer?).
Another frugal option, if you have a sewing machine, is to come on over to the Diaper Making forum and learn to make them from household fabrics, thrift store finds, etc.