I would say that there is no general rule, you should just charge what you feel is appropriate to cover your costs and that you can justify to your customers should they query it. Every business is different, so "admin fees" could relate to anything, but if you're charging them for admin there must be a reason in the first place? Just figure out why or what you're charging admin fees for and then base your pricing from that cost. Just make sure it's justifiable to the customer, that's the most important part.
Editor's note: I visited the site cited above. I caution small business owners to always visit the "About" page of a website. In this instance, there was no name, picture, or credentials of who is behind this website. When that happens, I then go to a site like "Whois Lookup" to see if I can find information on the place of the business. Here I did a find a registrant name but no address. I don't know about you, but why would you want to do business with someone who doesn't tell you their and where they are located! I published the comment because the advice is sound.