Are you talking about layers of a peel solution...say like TCA? If you are adding a couple of more layers of a peel solution like TCA the cost to you is not very much. You could cover your cost easily by increasing the price by $5. That would make your 4 layer TCA peel $45. Sounds reasonable to me! Remember to photo-document your work!
Well, no, not over-charging...keep in mind there are various price point out there. If you can justify your service price and your clients are happy, then all is well.
Since you don't masque with a TCA I think that you have a lot more leeway in pricing. I would probably do LED instead of masque. The clients that I have who can get TCA usually do it as an add-on to my Radiancy Express service ($65)
When you use more products, time or equipment your treatment price should increase accordingly.