If you want to launch for free and don't mind doing a bit of work for it you can put in at the Town Ramp at the town docks at the tip of alton bay. You have to put your boat in and then drive about a mile to drop off your trailer at the town DPW garage. Then you can disconnect from your trailer and drive back to the boat launch and park your truck there. When you get back from fishing you do it all in reverse. It is kind of a pain but it is free. There is a map at the launch that shows you were to put your trailer. The town garage is on Letter S road. Probably takes an extra 10 minutes at launch and an extra 10 minutes to retrieve. There is also another little launch at the tip of alton bay just before the bridge. Don't know the name of it but they charge $20 or $25 to launch. West Alton Marina is a good place also but like cody said it is $20.

Other free launch options are:

Meredith Bay. Launch at town docks and park truck and trailer in town lot just past aubochons.

I believe you can also launch and park at the center harbor town docks right now but not 100% sure.