Much of the pilot episode was reshot, so that inflated the first season's budget. My guess is that the second season will be budgeted at about the same initial level of the first season (about $50 million), but there'll be more money available for CGI and what not since the first season's budget paid for a lot of the standing sets that'll be used again.
The biggest question for me is what the direwolves will look like. The Northern Inuit dogs turned out to be a disaster - it was almost impossible to get them to do the slightest thing (hence why they've been seen so little in the first season) - and the older, much bigger direwolves are likely to be completely CGI. Fingers crossed that the CGI is of a good quality.