The problem you're going to run in to with spellcasting is that it power varies with level. At level 1, Color Spray or Sleep or what have you are nice for one or two combats, but they're unreliable. High stats and class features (maybe also medium or high BAB) will have better staying power and also have the potential to end combat quickly. Throw in armor proficiency and you have... a fighter, actually.

Proficiencies and attribute points only need to be bought once, so for minimal effort you could give 30-4(stats)-5(proficiencies), or 21 points per extra level. If spellcasting is a flat cost per level, then future levels should look about the same as the first one did in what you spend points on.

I hate to predict gloom and doom, but this sort of project typically has balance issues. Are you using d20 because you like it, or because you have not tried alternatives? GURPS is a widely touted point buy system, and it is supposed to be flexible enough for any genre. Shadowrun can be done with point buy and it has a cyberpunk+magic sort of setting. Riddle of Steel has a priority based system, which is like a more restricted point buy. It is supposed to be good for gritty, dangerous medieval combat.