Check out kbb.com for the blue book value. Then subtract some for the damage.
The shop isn't trying to make a quick buck off of you for the valve work, believe me, it is a bitch to adjust them, and $300 is about the going rate for valves, but the pain in the ass factor is worth it. I think the mechanic is just giving you some advice to make sure the bike is good to go for you. They are jsut looking out for you. If they were trying to make a buck they would tell you you need a "tune-up", and they wouldn't adjust the valves, just charge you $150 to check a couple bolts, the fluids, and throw some lube on the chain that took them 5 min to adjust, A.K.A everything that you can do yourself...MAYBE they will replace the plugs that they will charge you extra for.