Any price I would tell you would probably be wrong since I live in Texas.
I suggest you call a couple of Licensed Surveyors in your area and they can give you a price.
If all you want surveyed is property lines it will be cheaper than if you wanted a full survey showing building locations, elevations, and so forth.
Sounds like all you need is property lines and fences on the survey so you can see if the fences are inside, outside, or on your property lines.
They should drive iron pins at all corners and drive a wooden stake with a flag or other marker on it next to the pins. The iron pins will often be at or below grade so the wooden stakes will allow you to find them easily.
You mentioned "re-surveyed". If you have a survey that was done before you should be able to use it and a metal detector to find the iron pins that were placed during the original survey.
A single strand wire fence (barbed or smooth wire) might be a good idea for future reference if you don't want to go to the expense of a "real" fence.
Do NOT allow them to keep the trail on your property unless they are willing to sign a lease and pay you at least $10 per year. Failing to do so could end up with a mess later on when they claim title to that portion under "adverse possession".
I would recommend you have the survey done (or find the pins) and then go to them as nicely as you can and explain that you understand how such a mistake could happen, no real harm done, etc., but that your understanding of real estate law says that you cannot allow the trail to remain on your land.
A person who is on your land at your invitation, whether that invitation is explicit ("It's OK, come on in") or tacit (tacit being they come onto your land and you say or do nothing to indicate that they should leave) is called in "invitee" and you could possibly be held liable if they were injured while on your property.
HOWEVER! Do NOT just go out and plow up the trail or drop trees across it. That puts you in an adversarial situation immediately that can get out of hand. Remember, you have to live next to these folks and you don't need a feud going on with them for the next 20 years.
Be tactful but firm. And don't wait around until next Spring to get it taken care of. The longer they are trespassing the more they will think they have a right to be there.
Bill Tolle
Licensed Texas Real Estate Salesperson but not a Licensed Surveyor!