Giving my 20-cent's worth.
Journeyman:
This sure sounds like a rhetorical question, but I'll answer it anyway. A 20-cent shop is likely to get a lot of square action. Let's say the dime stores have a game at -130/+120. The 20-cent store will list it at either -135/+115 or -140/+120. Now, if they get more chalk money, let's say they raise it to -150/+130. Meanwhile the 10-cent shop might be getting balanced action at -130/+120 and not budge. If you liked the underdog, where would you bet? Would you still bet into the 10-cent line because a dime line is "better" or out of loyalty to that book? Of course not, in this instance there's no value.
This happens in a game or two just about every day during baseball season. So monitoring the books with 20-cent lines isn't so foolish.
Besides, in the real world, bettors don't just bet baseball. People might like other things a 20-cent baseball book offers in other sports.
The key is not taking the worst of it in any given situation.