like benzy, ive also found good deals on bullets and imr4895 powders that make it really cost effective.
i saw 500 m80 milspecs on gb for 52 dollars recently. its not uncommon.
also like dmazur said, sometimes its normal to dent the case mouths but it rarely hinders brass life. i start checking for casehead seperation after 4-5 loadings. more often than not its the case mouth that splits in protest during resizing, which is a pretty easy to read sign that its time to scrap it.
also try to find military brass, and even better, de-milled machinegun brass like de-linked 1919 browning machinegun cases. usually they are of the lc variety and are very thick walled and tough. ive seen them go 10+ times.
also consider that the condition of each individual rifle chamber will play into how long brass will last.
you will need to trim often, although if trimmed around five thousandths under spec, many can load twice between trimmings.