First- it's not "Extended Warranty". Mercury engines come with a Factory Warranty: For pleasure use, that's 3 Years. Anything beyond that is a SERVICE CONTRACT (Mercury offers two levels, GOLD or PLATINUM).
Product Protection Plans (Service Contracts) must be purchase PRIOR TO the end of the Factory Warranty. There are price increases per contract (surcharges) at the 1 year or 2 year Anniversary of original registration. If upgrading and extending, that is TWO contracts (and potentially two surcharges).
GOLD and PLATINUM are the two coverage levels.
Gold is quite limited in what it covers, and is considerably less expensive.
Platinum covers considerably more components as it adds much of the electrical system and fuel system.
The PRICING depends on what engine model, as well as when the Factory Warranty ends. Thus, quotes are Serial Number SPECIFIC.
There's a HUGE difference between buying 5 years of Platinum for a 40hp Fourstroke when NEW, and buying 5 years for a 400 2.5 years after original purchase.
Feel free to email me your serial number and I will be happy to provide a quote specific to your engine (including special BBC member pricing).