This is an interesting question. If you are doing a study on the cost of books, there are a few important considerations.
First, you need to account for inflation. Inflation alone causes the prices of goods and services to increase year after year. So, a history book that was published in 1930 costs considerably less than a history book that was published in 2010. What you can do is go to a used bookstore and look at the original prices of old books.
Second, since you are asking about history books, you need to make a distinction. If it is a popular history book, most likely the price will be relatively affordable. If it is an academic book, since there is a limited number printed, it will cost a lot more. This is an important factor in pricing.
Finally, if you want to get a sense of book prices now, go to amazon.com and do some comparative shopping. It will give you new and used prices.