If you current swim suit is wearing out, check out the deals from Lime Ricki Swimwear! I just recieved a promotion from www.livingsocial.com for a substantial discount. They have a store on Highland Dr in Salt Lake City, where they're clearing out display model swimwear for $5, top or bottom (more on that on their blog: http://www.limerickiblog.com/).
You can also order online, www.limericki.com, $8 standard shipping, and if you click on Factory, there's a substantial selection of girl's swim bottoms for $5 a piece (unfortunately that can't be combined with the Living Social deal, but you could use Living Social for a tankini top to go with the bottoms). You could get the whole thing for as little as $38, if I've calculated correctly.
I just recently got a new suit, but it's not as modest as I would like - these are much better.