I think this is part of my pathological customers thesis: you won't simply have more headaches at $500 than $5k because the customers are more evil about $500 invoices. You'll have more problems because _people who get into $500 consulting relationships are disproportionately flakes_. You get less flaky behavior from your $5k clients because people who buy $5k services are disproportionately not flakes; the $5k sticker price is kryptonite to flakes.

Bargain-seeking consulting consumers have irrational expectations for what $500 buys them. The $500 is likely coming out of a personal pocket or a pocket which they treat as personal, not out of a Budget which gets handled like an honest-to-God business. They are disproportionately likely to have overcommitted that $500 -- it's perhaps not their last $500, but your invoice is sitting with a number of other invoices and it may not be possible to settle them all simultaneously.

Many of my consulting friends move up-market and find that up-market consists of businesses run by disproportionately mentally healthy people who disproportionately have money in the bank and disproportionately conduct themselves with professional mien like paying invoices in a fashion that suggests their company has paid an invoice before.