level-based pay-as-you-go?
As close to this as I have seen is with the book
Buy the Numbers. It took all the elements of characters (feats, skills, saves, class features, etc.) and assigns everything individual XP values, so that you can buy everything a la carte.
It's sorta level-based in that you're capped by your # of HD as far as how much of a given thing you can have -- e.g., stuff like BAB, CL, etc., cannot exceed your HD, and so forth (btw, even HD are purchased like this).
Oh, and it's done under the 3.x OGL.
It really does function much like a skill-based buy-as-you-go system, but still maintains the "level" feature.
I do own the book; but I haven't had a chance to use it yet, nor have I had the chance to tear everything apart to see how well everything is priced (as far as balance issues go) -- I do happen to know, however, that it costs more to build a PHB caster than it does a PHB martial class.
The biggest downside that I noticed right off the bat was that it really only had Core material covered; however, it does give some guidelines for pricing new abilities (such as from PrCs, etc.).
I'm considering using this for my next game .... I'll see. It's gonna be an E6 game, so I'm a little concerned about variety.
(I was just going to use Generic Classes from UA; but I've gotten a special request for Buy the Numbers from one of my players, so it's up for consideration).