Over on Swimmingly, Molly Fitzpatrick reached out to every MLB team and figured out how much it costs to publicly propose at each MLB ballpark, as well which services each ballpark offers to make your awkward public proposal as magical as possible.
For $38.50, the Pittsburgh Pirates will display your proposal message on their scoreboard, the cheapest available option. The Dodgers are the most expensive at $2,500, for which you get your proposal shown live on the scoreboard. There's a limit of one proposal per month, although Swimmingly does not specify if this is per-person or otherwise.
For 500 dollars, the Marlins will display a message/video on the screen, throw you a PA announcement, and have Billy the Marlin deliver a dozen roses. Does it really count as a public proposal though? Bad deal. Go read up on all the other ballpark options on Swimmingly.