there are subs and there are subs of subs and theres a big difference between the two. a sub from a large company was probably required to foresee about 5 years of tree clearance during your install, but im no tree growth expert myself so 2 years is not too bad.. having seen the tree issue he or she would probably have placed the dish out on a pole as you suggested and then dug a 100ft trench. this sub is not going to be able to charge you anything as he is getting paid a set amount by his company to do the install, except for the actual trench which may be 1 to 2 dollars per foot after the first 25 feet or so.. your fee for the service call will be a set amount and so will the sub's except for the trench part, unless you tell them otherwise and get some type of upgrade. a sub-sub is another story and the fee will probably be a little higher considering everything he or she gives you was bought out of his or her pocket.
you have to consider that whoever it was, may or may not have seen the issue but probably wanted to get paid for the job.. sometimes customers don't quite understand that with heavy trees satellite signals are a privilege.
you also have to consider that the fee they are placing upon you is a direct result of you turning down the personal protection plan.. if you had the plan im pretty sure they would have moved the dish for free.. otherwise you are always going to be charged to have them roll a van out somewhere with materials to be burned. directv in particular is the largest satellite tv dealer.. they want your money but they really dont need it.
i would personally consider that moving the dish yourself probably wont be worth it, unless you want to ensure the best line burial possibly, as a 100ft is no easy trenching. putting it on a tree is not a good idea because trees slowly grow. putting it anywhere other than directv's standardization will result in the tech having to relocate it anyway during a service call if it is ever QC'd by the subs supervisors. if you dont have a satellite meter then you would have to play that game of having someone stand in front of the signal meter screen while you move it all around, and with the HD dishes that just aint going to work buddy.. at that point after some time of weathering u may end up having to call them back yet again to have a tech come re-peak your dish where you will be charged yet again.
having said all that i'd say the answer is simple, pay them the fee to have them move it, understanding that its not quite a fair situation, but considering the actual options the price is quite a fair deal. cancelling the service and going with any other dealer is probably going to cost you much more, and possibly more of the same hassle.