True, but also don't fall into the trap of incapacitating paranoia, either. It's kinda an interesting point though, since when similar situations (for publication authorship, not a monetary transaction) come up there is usually a better established structure around the resident, from departmental to institutional support/advice.
I really don't know what a fair price would be, especially for a resident. You could bulk them or charge by the slide, lot of ways to go with it. Talk about 88305 is all well and good, but probably isn't a fair or direct comparison. If you're just getting a bunch of slides and are only being asked to drop a number for inflammatory score for each one, it's a pretty limited histologic evaluation. I did something eerily like this as preliminary work for a publication -- come to think of it, no idea whatever happened with that beyond some local presentation by a surg resident -- but since it was for publication, money was never an issue.
The problem with ballsy negotiations is that if you ask for $400/slide and they have 1000 slides ready to ship to you for what amounts to <1 minute work per slide once you get a system going, then you'll probably never hear from them again as they go find someone willing to do them for $0.40/slide. Or vice versa if you say you'll do it for $0.40/slide and they're willing to pay $10+. It really, really helps to have some idea of the market before you say something silly to one extreme or the other. My unfounded suspicion is that you'll be looking at a few hundred to a few thousand bucks total -- otherwise if they're going to spend that much they might as well use someone board certified (and who carries appropriate insurance).
If you have someone local you can trust (i.e., won't raise a big fuss for no good reason or try to jump your claim for the job, but has been around and knows what they're doing and what you might be getting into) to ask the question, I'd do that.