How can I calculate the mpg for a 2004 Hyundai Elantra towing about 2,500lbs?

Regular MPG's are around 27. Towing a trailer I would estimate a decrease of 30-40%. Figure around high teens for your budget. Keep it slow and allow plenty of room for braking.

Double check that weight rating with Hyundai. Start with the owner's manual. I am betting that the weight rating you were given came from U-Haul. They have their own rating system that has nothing at all to do with the factory ratings on vehicles. Their system says any vehicle can tow up to its own weight, but no more. I saw them rent a 3,500 trailer to a guy with a Windstar (rated for 2,500) and try to deny me a 5,000 pound trailer for a Grand Marquis with towing package (rating 6,000). My bet is that your Elantra is actually rated for no more than 1,500, is that.

Actually, that is the rating from Hyundai themselves. I like to make sure I do all my research first, hence why I thought it might be helpful to ask all of your professional opinions. Thank you both for your help. If anyone else had other thoughts, they are welcome as well

In Europe, it is rated for 1,250 kgs. That is 2,500. In North America, it is not rated at all. If you are involved in a crash, at fault or not, your insurance may well be null and void because the car is not rated to tow the trailer attached. Also, watch your weights. There is a little under 500 kgs between the curb weight and the gross vehicle weight. The weight of all passengers, and what ever else is in the car, PLUS the tongue weight has to be withing that 500 kg window. You could overload the tires or stress other parts of the car by exceeding this rating. It may cost more, but for safety, you would do better to rent a truck and tow the car. For one, the clutch is going to take one HELL of a beating, especially if there are hills on your route, and replacing the clutch is going to be more than the cost of the truck.