Hello, good evening and welcome...
I don't think that is a bad cost estimate, IF all you need is seals. Old cylinders can present other problems that require additional parts to properly correct. Did the shop actually see the cylinder?
Lets look at it this way... if you can get the cylinder open and take a look at what is involved in re-sealing then you can be the judge as to whether this is a job for you. It's not rocket science but you might have to be resourcefull. You may not have the necessary tools.
The average seal kit for my backhoe is around $45.- retail, the dealer mentioned 3 hours at $75.- per for labor. That probably includes removing the worst cylinder to get at.
I figured it wouldn't be any more broken if I tried but couldn't do the job. It doesn't take 3 hours to re-seal. I didn't meet with any insurmountable difficulty.
You must be the judge of your abilities.
All the best,
Martin