BearClaw -
If you watch Cori closely, you'll get a feel for when shops are about to go up in price, by how much they'll increase, and what price they "go for" given the specific location or general area. Don't ever, ever, ever take one at the starting price which is available for future months. They usually go up for the first time on the 1st or 2nd of the month. The first increase is to a price determined by a percentage of what the shop finally got taken for during the previous month. After that the frequency of increase is client specific, and the percentage/dollar amount of the increase is location specific.
Your $13 shop must have gone for a lower price last month than your $19.68 shop did. It may have to do with the poor conditions of the more expensive shop so that those who know the location don't want to do it, or because that location is more remote and it takes more $$$ to lure someone to its location, or because someone flaked on the more expensive shop last month, so when it finally went back on the board last month it did so at a higher price later in the month, or because a desperate or impatient shopper or a newcomer who didn't know the system unwittingly sold themselves short on the $13 shop last month, getting paid less themselves last month and driving the price down for when you wanted to take it this month.