Obsessively search craigslist in areas where rafts get resold (Billings, state of WY, Denver area, etc.) and be ready to make the drive. I think you'll eventually find a bucket boat and a minimal frame for about that much.
My buddy has a Saturn with a cobbled together fishing frame and it does the job. He's had no issues with the raft, but different year Saturns have different issues, from what I've read on here.
Not a great brand, but this would get you on the water:
No mention of brand, but a great buy if it doesn't suck:
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