What would b the mid-point for open cles in statistics - say for 10-19 the mid-pt is 14.5, what about 100+?

Say for listing the salary of persons, the cles are (in Rs.) <1000, 1,000-10,000, 10000+. There are 3 respondents in each cl. Now for calculating the TOTAL salary we must have the AVERAGE salary per cl - Its ok for the middle cl = average (1000,10000)=5500. What about the 2 extreme cles. For simplicity I just take the points directly - e.g. mid-pt of <1,000=1,000 and same for 10,000+; the mid-pt is 10,000. But is it really statistically valid? If so or if not, why? And what other alternative ways are there of calculating the class mean of open classes? Any particular statistical references (if possible from the Internet with the web-page links) would be very helpful.