So am I following that your dad is going to buy you a bike for your birthday, but you're not specific about how much he's going to pony up for the purchase? How, exactly, is this going to go down? Is he taking you down to the LBS and you two will work it out with a sales person? Is he going to get with you and you give him a price - he goes to a website and punches in the CVC and expiration date (we know you got the CC No. already memorized)?

If the last option is the game plan, why not show him a series of $1200 bikes, knowing they're above the price he's going to spend, and then counter by showing him a "pathetic" $800 bike and seal the con with a "I'd settle for this" after he tells you to piss off on the $1200 bike. The oversell always works, just make sure you're showing him something that, lord forbid, he jumps and says "fine, you want a $1200 bike - let's buy it," you actually want.

tl;dr

bait him with a $1200 bike that you've hand picked for safety
Hook him with a $800 bike you know he'll see as a steal.

p.s. look for outdated models - 2009-2010 - which shops are working their asses off to get off their inventory lists.