I charge at three price points for 35mm negs, and this is based on the degree of handling required during scanning.
The easiest to scan, is consective negs in one strip. Next, comes select frames on the same strip requiring jockeying the strip to locate the frame. lastly, single frames take the most time, as they require individual placement in a separate holder.
Unlike others, I don't just run a strip with a single auto adjustment. I tweak each frame following the scan – colour, exposure, shadow/highlight... I'm out to deliver the best results, so consequently my neg scanning prices ain't the cheapest.
To answer your question, I charge the same for a strip neg scan, as a single 35mm slide. Scanning resolution and other options are pretty much the same for both slides and negs.
Check out my site if you like. It's in my ehMac profile.