I recommend for you to look through the publication in the following link which contains 12 items to be considered when calculating the income from tourism:
JUST GO throught the book of Douglas Pearce (1981): Topics in Applied Geography, Tourist Development. Longman london New York. he has discussed concept of income from tourism and even about multiple income and multiplier effect even he has given number of techniques
In the United States, the rule of thumb for Heritage Tourism, is that for every $1 spent in a local economy, that dollar will change hands from eight to 15 times. To be on the safe side, I usually round it to $10 for every dollar spent by a tourist. The talk I give to local leaders is if a visitor spends $5 on a sandwich and cup of coffee, that $5 has a $50 impact in the local economy. That does not sound like much but when one multiplies it by 1,000 visitors, that's a $50,000 impact and by 100,000 visitors, a $5 million impact. Further, tourism may not create many jobs (depending on the site) but will help preserve existing jobs which is just as important to the local tax base. This is just a ballpark estimate. The Virginia Tourism Corporation helps estimate tourism reveneus for the state: http://www.vatc.org/research/. (We're very big on heritage/history tourism. Our estimate is American Civil War tourism is a 2 billion dollar industry.) Hope this helps.