There are a few variables to contend with regarding how long a single wig will last,some of these being:
1/The quality of the wig
2/ How you treat the wig,especially when washing etc..
3/ What kind of enviroment the wig is subjected to daily.
Generally you can get a good 3 months out of a wig but it is very much possable to get 6 months or more.
How often you want to de/re your wig depends on yourself and your own preferences.So there is no hard and fast rule for that but for hygene reasons 2 wekks and above can get messy.
I change weeky with a full adhesive head bond and have no problems.
As for expensive?
That all depends on where you buy from,again that is up to you but it is worth bearing in mind that because you pay big bucks for a wig doesn't mean you will recieve a better quality wig,unfortuanatly it doesn't work like that and there are some unscupulous dealers who sell way over overpriced wigs.
Your best bet is to shop around and settle on the place that gets you what you want with good customer service.
There are plenty of references on here to help you decide where to buy.