When you register your boat there are 2 main costs called out on the form; boat fee and registration fee.
The boat fee is based on the type and age of your boat and declines as your boat gets older for 5 years then remains the same. Minimum $10.
Here's a pointer to the fee schedule (not sure if it's current)
Boat fee
For the typical 20 odd foot cruiser on the lake it starts around $200 - $300 and declines to around $20 when the boat is 5 years old.
The registration fee is based on the length of your boat.
--- Up to and including 16 feet -------------------------- $12.00
--- 16.1 to 21 feet ------------------------------------------ $17.00
--- 21.1 to 30 feet ----------------------------------------- $26.00
--- 30.1 to 45 feet ----------------------------------------- $36.00
--- 45.1 feet and over ------------------------------------- $46.00
Additional costs in the registration fee total

current?)
--- $5.00 fee per registration for the milfoil prevention fund
--- $1.50 fee per registration for the lake restoration and preservation fund
--- $1.00 fee per registration for the search and rescue fund
--- $5.00 fee per registration for the statewide public boat access fund
We are also charged an agent fee for each registration.
Further the boat fee portion, if paid to a town agent, stays with the town. If you mail it in to the state, the state keeps the fee. The registration fee portion goes to the state.