Generally, you figure only loaded miles, because you could be hauling something else back.
The going rate starts at .60 for some commercial haulers and goes up to $2.- for private ones, depending on so much else than just the miles, like if you are a trainer hauling to a show for your clients, etc.
Be sure you have proper insurance to haul for pay, or if something happens, you may be in trouble.
For my clients, I charge:
* Loaded – 1 horse $1.25 per mile
* Loaded – 2 horses $0.75 per mile per horse ($1.50 total per mile)
* Loaded – 3+ horses $0.60 per mile per horse ($1.80 total per mile)
* Unloaded $1.00 per mile
(we're in California, truck is a diesel dually, and I'm hauling local - to horse shows, clinics, etc)
I would still charge for the unloaded return, since it's still using fuel (w/ trailer still attached), my time, & I wouldn't otherwise be going there. That's why the "unloaded" rate is lower.
However, please be careful. Are you a trainer with insurance, or just a friend willing to help out? Do not take any $$ if you are friend - or at least don't have a verbal/written agreement about providing a service. If you don't have insurance & are covered for trailering other people's horses, you could get stuck in some serious problems if an accident happens.
Most friends just load up together, don't say a word about the "arrangement," and maybe a $20 bill shows up on your dashboard at the end of the day. If this person really needs her horse hauled somewhere, I'd leave that to someone who hauls for a living / has insurance to cover the situation.
A few years ago, the going rate was about $1/mi. They charged for the round trip. Another person charged $2/mi, but I did not use them.
Some people also have "hookup" fees - so a flat rate just to trailer and then charge by the mile as well. Might be something like $50 hookup and then $1/mi. That may only be for a loaded mile though - which may be why there's a hookup fee - to cover gas for the trip back and some wear and tear on the vehicles.
I think there are several factors involved as one of the other posters mentioned - if you charge (as in to earn money) then you may be required to carry additional insurance (and might also depend on the weight of the trailer) vs. doing a friend a favor where you would essentially be reimbursed for cost. You might check the current IRS mileage rate to see what it is - I think its around $0.55 per mile. The IRS factors in insurance and wear & tear on vehicle. For example I recently got reimbursed for mileage thru my job. I got a check for $20 for 37 miles of travel. My car gets about 24 MPG so my fuel cost was probably about $6.00 - so the IRS rate is $14 more than my approximate fuel cost.