I'm looking to buy a 6505+ for a low price. The catch is that the standby switch is "out" according to the owner. What is the worse case scenario and how much would it cost to have this repaired?
Worst case scenario? The guy selling it thinks that "standby switch out" means that the amp won't turn on; it turns out to be the power transformer, and you have a brick.

We need more information. If the amp is stuck in the playing position, and it makes sound and just won't mute when the standby switch is used, that's super easy to fix. If it cannot be turned on, I would not touch it. You can't assume it's just the switch in that case.

Ask him how he knows it's just the standby switch, and exactly how it's (not) functioning.
All he knows is that the amp is "stuck" on standby. Because of this, there's no sound. I'll make sure to have my tech check it out before I buy.
Good plan. Hopefully it's just the switch, but it would be awful to buy one expecting that issue when something much more expensive was broken. Unfortunately a blown output transformer could also have the same symptoms, so your tech's input will be a great asset. Good luck!