It'll cost you $450 to propose to your beloved at a Phillies' game this year. Considering the quality of the team (yes, Ruben, major league clubs really do need bullpens), there are probably better venues in which to pop the question. Jack in the Box comes to mind.
In any case, should you be determined to risk rejection of all your life's hopes in front of 30,000 strangers, I'd suggest that for $450 Chase Utley should agree to be your best man, Larry Andersen should offer toasts at the reception, and new TV announcer and solid citizen Jamie Moyer should agree to be godfather of any children issuing from the union.
For another $4000, I understand Chase will agree to be father of your kids (with your wife's written consent, which should be easy to obtain), so your lil sluggers have a better shot at MLB careers. Hell of a return on your investment if it works out.
The most expensive ballpark for proposing is, not surprisingly, Dodger Stadium, where it'll run you $2500. No verification on rumor that this includes a lap dance with Tommy Lasorda (sorry).