There's a very simple way to reassure yourself if your BO is gouging you or exploiting the hay shortage.
Call around to other boarding barns and hay dealers. Ask the BMs what they paid for hay last year, what they are having to pay this year and if they are having to use other, out of area sources to get hay at all. Factor in the extra delivery cost if the latter is true. Then call hay growers and dealers in your area and ask the same questions.
Even if you board went up more than the price increased, consider that your barn's other overhead prices may have been creeping up (Diesel is up over $4./gallon, and grain prices are high as well.) and your BO may have decided to do one increase all at once, rather than several.
Golden Horse made a good point, this is a legitmate question from someone without a lot of experience. The cure for that is to do some research and get some experience. It might also be instructive to call around to some other boarding barns and find out what they charge for similiar facilities and services.
Boarding barns are often little hotbeds of gossip and grievances, and a couple of complainers can get everyone all stirred up at a perceived unfairness. Find out if it's reasonable or out of line by doing some research of your own.