$500 for the STC
$7500 all-in for them to convert it for you
The other STC is called a "texas taildragger". Google search will return lots of results for Cessna 150 tailwheel conversion info.
#1. Choose an early model 150 with manual flaps. They like most early Cessnas have a much lighter empty weight, more rudder authority with the square tail, and popping the flaps will come in very handy. If you paint some windows on the side, people might mistake it for a 180 or 185 from a distance.
#2. Strip everything out of it. Interior, carpet, passenger seat, vacuum system, radios, nav instruments, lights.
#3. Install every mod you can to increase wing area and gain STOL performance. VGs, leading edge cuffs, wing tips, wing extensions if available. Rig the ailerons on the droopy side of neutral (but within limits of course ) if a Robertson STOL is not in your budget.
#4. Install a climb prop or re-pitch the prop for maximum static RPM as listed in the Type Certificate or get an STC to install one. Only the 150G and later have a seaplane prop specifically in the Type Certificate Data Sheet that allows a static RPM of 2600.
#5. Cool days, low elevations, long lakes, float rolls, and boat wakes are your new friends..... since you won't be able to take friends along.
For the same money you can get a C 172. It is an honest 2 place float plane with a comfortable cabin and it does OK on the Edo/Cap 2000's. With the wing extensions and bigger floats it actually performs surprisingly well and is IMO a nicer flying floatplane than the C 150.
One thing is for sure, stay away from a 100 hp C 150 on floats. It is grossly under-powered and is a total dog.