A new tank cost me $88, to repair the saw a groundman failed to move after handing me a bigger saw. The intake boot, which used to go bad, should be replaced while the saw is apart.
Get him to let you test it, to see if it has a leak. Take it to a safe place, say over a bucket, and fill both tanks with gas, one at a time. I reckon you could use water, if you plugged the intake gas line at the carb. It could be dried if you had some time. Wouldnt matter if you find a leak.
Your results will tell you what its worth. I'd say 50-150 broke, 150-225 if it is OK. If the gas cap is green or gray, it is a newer saw. Also, look for date of mfr, next to serial #, below bar mount area.
Figure on a hour of labor to r and r the tank, more like two since you will want to take your time. The magneto can stay on, so the flywhell gap will stay the same, it just makes for a tight squeeze. A few sharp taps will spring the flywheel free of the crankshaft.
It is not a hard job, if you are careful, and all you really need is an allen wrench.
My shop guy never replaced the oiler on my three first generation saws, he just worked on the whole system to get it free flowing. now mine all work consistently, just flow a bit on the light side.
Only real problems with the saw is the temperamental fuel system, and the chain brakes, which still get floppy after a year or so of hard use. Always got my first one free, don't know about now, two more are bad.
Impatiently waiting for the two new models (336 and 339?) to come out.