I did one in January--got a nice mild day, so it worked out--and as far as I'm concerned, winter is the time to do them. Nice clean snow, no flies, no dirt or mud or dust, so almost no chance of infection, and the cooler weather makes the surgery a little easier on them I think. You just need a mild day (we wouldn't do it on a -40 day of course!!) and a place to drop them out of the snow...of course in Texas I guess snow isn't such a good option for you????
I also had 3 done in April this year, and that was okay--it was dry, still no bugs. Previously I've usually done them in the fall--September, October or early November. There's no way I'd get one done now, even if we didn't have mud like we've got this year from all the rain....there's just too many bugs in summer, and I don't like doing it when it's really hot like this either.
As for cost here--the single one in January was about $115 with mileage included. When I got the 3 done in April my total bill, mileage and all, was $277, so that worked out to $90 each.