Got $20? Then with a little savvy you can hear — and glimpse — the greats doing their thing. A seat at a table at The Blue Note runs, for most weeknight shows, $35. But a seat at the bar at the club’s entrance, off to the right of the stage, generally costs you a double sawbuck. (Prices go up according to a simple methodology: If the headliner is someone you can imagine having turned up on The Cosby Show — say, Chick Corea, or Kenny G — then any seat will cost you, but the table/bar breakdown is proportionally similar: $75 to $45 for Corea.) There’s a trick, though: You can’t score a barstool in advance, so to catch Lou Donaldson, or Donald Harrison and Eddie Henderson, or that ridiculous Odean Pope/Pharoah Sanders/James Carter combo, for about one-fifth of the price of your monthly MTA pass, you have to show up early. Doors open at 6 for the 8 p.m. show and at 9:45 for the 10:30; we recommend popping your head in and then, if there are plenty of seats left, you can duck out to John’s Pizzeria.