The figure of 500K was relative to the previous posts. Personally agree with you and believe it would be optimistic to think we'd receive such a sum. I think we'd do well to touch 250K but i don't manage compensation fees.
As for asking me to name players we'd be able to bring in that isn't difficult. Just look at the League Two top scorers last season. For instance, John Akinde, Matt Godden, David Wheeler. All three play for clubs that are unlikely to turn away from a six-figure bid. All average around 1 in 3, with the exception of Akinde, who's ratio was closer to 1 in 2. Wouldn't say they are any better or worse players than Collins, therefore you would assume their respective clubs wouldn't ask for a fee beyond Luton's reported bid.
Matthew Godden is coming into the last year of his contract. David Wheeler's contract was undisclosed but it seems to me that it expires this summer. I would say that qualifies as available, wouldn't you?