Priceline (Alamo) accepted my bid of $10 per day ($169 total) for 10 days for a standard-size car on Big Island in late May. The cheapest reservation I could find elsewhere was about $80 more using the BJ's coupon code (on these forums). On TripAdvisor, others are reporting similar Priceline successes ($10/day).
Good news! I'm going in May too!
I'll soon get moving on Priceline for my Feb./Mar. trip and report what, if any, success I have.