Anyone have a good comp for a AP rune page? I already have a AD one, and I want to build an AP rune page.

AP runes are very variable. My suggestion is that you go on www.mobafire.com, look up the champion that you play, and use the runes there. But for a general AP rune page I suggest:
Marks of Magic Penetration
Seal of Armour (for those tricky Talon mids)
Glyph of Magic Resist (since you'll be laning mid agains another AP champion)
Quintessence of Ability Power.

The seals are very variable on what champion you take and what you fight against, so feel free to swap them out.

Quintessence: movement or flat ap
Seal: armor or flat/per level mana regen or hp/level
Glyph: flat/per level ap or flat/per level MR
Marks: magic pen

Between the different options will depend on the kind of champion you play, and who you are laning against.

Keep in mind that these are not set in stone, just the more popular ones floating about. Experiment between as many as you like, and find what works best for you.

Each AP is going to be a little different. I wouldn't build a Veigar and Fiddlesticks the same I would Evelyn and Singed. I would give the Seal of Armour, as Fate said above, to Evelyn and Singed, but not to Veigar and Fiddlesticks. I would give the Seal of Scaling Ability Power. Same thing with Glyphs, I would give Veigar and Fiddlesticks Glyphs of Scaling Ability Power.

Everybody does things different. That's why you should go to places like www.mobafire.com and www.lolking.net see how other people do it. Make sure to also look at the discussions to see how people argue on way or the other. Then you will get enough information to make up your own mind.

I'll detail some ideas for AP rune sets below.

For Marks, Magic Penetration runes are by far the best choice as spell pen is harder to itemize in-game than AP. Less so in season 3 since the new items but still, the early game damage increase is very significant. Most AP casters will also rune up Magic Resist blues so the spell pen runes are almost a necessity to counter your lane opponent's MR.

For Seals, there are basically 3 choices; Scaling AP, Mana Regen (flat or scaling) or flat Armor. Scaling AP seals are well suited to burst casters with huge nukes such as Annie or LeBlanc and even work well for DPS mages like Karthus or Cassiopeia. Mana Regen runes are a niche pick for champions with early mana problems like Orianna. Flat armor seals are useful in two situations; the first is when you are unfortunate enough to run into a Talon or Lee Sin mid. The other is when you are playing an AP top lane against a bruiser like Darius or Pantheon. Flat health seals are 4th option but I would only recommend these for one mage; Vladimir.

If you are playing an AP mid, I would almost always recommend taking MR blues (flat or scaling). Flat MR blues are better against mages with a lot of early game damage (Annie, LeBlanc, Cassiopeia, Sion). These champions are geared towards getting fed early game through kills rather than farming their way to victory (like Morgana or Karthus). These runes will help you deny them those early kills by making you slightly more durable. AP/level glyphs are also fine if you know that your opponent does not have a lot of harass. For example, if you're playing Swain against Nasus top, you're not so worried about taking damage as much as not allowing Nasus to farm. AP/level glyphs would be fine in this case.

Finally, for quintessences, there are really only 2 main choices; movement speed and flat AP. Flat AP is a safe bet for mages who have a built-in gap closer (LeBlanc, Akali, Ahri, Katarina, Kassadin etc.). Movement speed is essential on melee mages (like Cho'Gath or Sion) or champions with inherently low movement speed like Ryze. Ryze is also terrible at pushing lanes so roaming comes with the job. Flat HP quints are also very good but again I would only recommend them for Vladimir as they are exclusively geared towards early game survivability and the +80 odd health does very little late game.

Flat AP Quints
Magic Pen Marks
Mana Regen Per Level Seals
Ability Power Per Level Glyphs