7.5c per Copy isn't too bad, we charge roughly the same for our contracts. They do charge per copy because it is actually to recover some costs incurred. Profit on that 7.5c is most of the time 1c or less. Im not too familiar with that machine, but reason it is a "refurb", is because it was a machine another customer had a contract on and has either expired and not been renewed, or was cancelled of sort.
Just make very sure you look at what other items they include on the contract, mainly service items like the cleaning blade, fuser unit, feed rollers etc. Also make sure that they let you know about developer, if its a cost to you or them. Its a black powder similar to toner, but helps the transfer of toner to paper.
The advantage of the service contract is the quick service you should be getting. One normally states a response time on your calls to customers. The other bonus is the toner which isn't paid for by you. Now do keep in mind its a way of getting you to not care about how much you copy or print, seeing you pay cost per copy. But I have seen companies charge a customer for toner and cost per copy.
On to the HP machine....Yes, ANY cartridge not approved by the manufacturer does void your warranty(its written in the documentation supplied with the machine). These tend to run a lot dirtier as well as your prints coming out a lesser quality than with original supplies. SO it wouldnt be recommended to use those. You would end up having your machine serviced more frequently as well.
Do keep in mind the page yield is set at 5% coverage on all toners. So calculating profit or cost in a internet cafe environment is going to be nigh impossible, but on the average you could get roughly 1000 prints on a 7k yield. Depending on the amount of "text" on a page. You might even get less than that some months, others you could get more. They do come with a drum attached to the unit, hence the high cost of the replacement. When using the refill or refurb toner, keep in mind that you're using an old drum unit and you do not know how many copies has gone through it. So that drum could fail while you have for instance 40% or more/less toner left. Servicing of the HP machines is not included in the warranty either, so you basically only have support on the machine if it is in actual fact faulty.
I dont condone generic or refills because im the poor bloke having to clean the mess.

lol