The price you pay often comes down to how much of a share of the box you have. As a Moodle Partner we have relatively low numbers of sites per server than you might find on the average shared hosting. In addition the box is dedicated to hosting Moodle.
However, if you only have a few users it doesn't matter so much and low-cost hosting may be perfectly adequate. At the risk of stating the obvious, you do have to make sure the hosting exceeds Moodle's basic requirements. Which very cheap shared hosting often does not.