They should pay you for the easement and it would also be reasonable to ask them to pay the cost for you to have your own attorney review the easement. Your attorney should advise you about the wording and your rights -- and responsibilities -- under the easement. One thing the lawyer should address is your question about repairing any damage - that should be spelled out in the easement agreement. As for how much they should pay, it is really a matter of negotiation and I am not aware of any formula or rule of thumb.
If you come to an agreement on the terms of the easement, an attorney should prepare a written document describing the terms of the easement, which would then be recorded in the official records of your County, just like a deed. It will then be part of the title to your property, so that anyone who buys your property in the future would be aware of (and subject to) the easement. This is why it is very important to have your own lawyer review the terms to be sure you understand and agree with them.
You are compensated however you arrange with the buyer. You can ask for whatever you want and impose whatever conditions you can bargain for. However, there is a legal prohibition against having a landlocked property. So if they have no other way to get onto their land, you may have a problem. Particularly if that lot was once part of your property.