Existing Point-Buy d20 Systems

Several such systems already exist - most notably D(liberation)20, the Anime SRD, Mutants and Masterminds,  Complete Control, and Eclipse: The Codex Persona.

D(liberation)20 attempted to turn each ability into a feat. This did not work well since some abilities (for example, a Paladin's charisma-based bonus on saving throws) were a great deal better than others (a Monk's +2 versus Enchantment spells). It can be found with enough net searching although it was abandoned long ago.

The Anime SRD attempted a direct conversion of the Tri-Stat system. There's an examination of why it failed to work: Anime SRD evaluation. It's also long abandoned, although it too can still be found on the online.

Mutants and Masterminds works fairly well, but is designed for a mid-to-high-level superhero game. If you wish to play other types of characters it does not work so well. It is, however, well supported although you do have to pay for it.

Complete Control attempted to set things up so that the various abilities were purchased with experience points. It also does not work well, and again here's a breakdown of why: Complete Control Breakdown. It still has an active forum though and is still available, although you do have to pay for it.

Eclipse The Codex Persona more closely resembles the Hero System or GURPS in style; abilities have a variable point cost and you get so many points per level. It's the only point-buy system I know of that is back-compatible with 3.0, 3.5, Modern, Future, and - for that matter - Iron Heroes and Pathfinder. It is still supported and is available as freeware, although the author does ask that you pay for it if you like it. ,