Mark is regularly depicted buying ridiculous expensive luxury items, such as buying a sex doll to use as a crash test dummy which he reveals was $6,000. His income as revealed in the concept and theme song is only $3,000 per month, as in the doll was two entire month's income, which only works if he bought absolutely nothing for two straight months. Mark receives nowhere near enough money to be able to buy all of the expensive things he does as easily as he does. (It's clear that Tammy doesn't support or know about these shenanigans until revealed to her, and all of her income would generally go towards actual legitimate expenses). An occasional big expense would be believable, but this is every episode.