Okay thank you. What's is a MPEM?
More or less, it's the "brain" of the jetski. A sealed, non-repairable unit where all the electronics go into it and all the signals come out of it, example all the sensors, speedometer, etc... I can't say how much your specific MPEM might cost, but I've seen numbers in the $250 to $350 range. Some are more rare, and therefore more expensive, than others.
You can destroy your MPEM by doing things like applying 12 VDC power to the battery while it is connected to the Seadoo. Always recharge your battery off-system. Other people sort of "poo-poo" the risk of this, but I'd rather be a nervous nellie and not have to drop $300 on a part I might have preserved if I had simply taken the battery out of the Seadoo.
Is your start button all gummy and trashed-out? It's possible it's pushing constantly on the waterproof switch underneath it. It's also possible your solenoid is shot. That's like a $25 part, so if somethings broken, you want it to be that. Starters are in the $200 range. MPEMs in the $300 range. So hope that if it's broke, the part that's bad is one of the cheap ones.
EDIT***
I'll also add that if you are going to work on it yourself, it would be a really good idea to download the Parts Catalog for your particular model & year Seadoo.
http://www.seadooforum.com/manuals.php