No real info BUT
I have no real numbers, which is what you are asking for, BUT I see a few Alpaca farms around here and they are all the "hobby" farm type. I do know there is a tax incentive for raising livestock. My assumption (yes, I may be an ass because of this) is that these owners want the tax break for ranching, but don't want to mess with big animals like cattle or horses. My thinking is that the couple decides that Alpacas are reasonably sized animals to have around, so they can be handled easier.
For numbers, look up what property tax rate is for your county for ranch land and see how many animals are needed to qualify. I believe ours is 5 animals and you need more than 2 acres. Chickens fall into a different category. Anyone can have them around here. Also look into how much Alpacas eat and how much food costs. Add in some veterinary expenses (rabies shots at least) and the cost to house them (pole shed, some heat in the winter) and see if there is any savings from the tax rate you pay now. As for selling the wool, I have no idea what going rate is, but I know that knitting is super trendy right now (my wife and her friends do it) so it probably is at it's peak (for financial planning purposes, don't count on that rate every year).
We have more horse farms than you can shake a stick at. Why people love horses so much is beyond me, but to each their own.