Dates of the Spring Festival, etc. - will travel to Mt Huangshan be more expensive then?

Well, as far as I know, the ticket fares are CNY 230 from Mar. to Nov. and CNY 150 from Dec. to Feb. of the next year.
The Spring Festival holiday of 2016 falls on Feb. 7 - 13, so the ticket fare will be CNY150. However, the fare of accomodation and dinning will be more expensive than usual time. Or you can just go there a few days earlier or later than the holiday period to save money.
About the weather condition, the temperature is 16 F to 27 F. Although there are many sunny days during that period of time, it's so cold. You need to bring your overcoats, jackets, sweaters, pants and boots in order to keep warm.