Those tubes are also known as EL84s. They are very popular in electric guitar amplifiers - especially those looking for Brit pop or Beatles type of tones, but they were originally brought into the world as tubes for radios.
The prices vary because the old American (GE, RCA, Sylvania, etc) and European (Philips, Siemens, made 6BQ5/EL84 tubes are considered superior to current Chinese (Shuguang or TAD brands), Russian (Sovtek and Mullard brands) and Czech (JJ brand) made versions. You'll see the term "NOS" used a great deal. It means "new old stock" - a tube made back in the last century that was never used before. Many folks will tell you that an RCA, GE or Philips 6BQ5/EL 84 manufactured back in the golden days of American and western European tube production will sound better than a currently produced tube and will last much longer.
The 7189 is a more rugged version of this tube that can handle higher plate voltages (B+). A typical 6BQ5/EL84 is supposed to handle a B+ of around 300 volts. A 7189 is supposed to handle around 400. If you find someone selling a 7189 set, they are probably going for more than even the premium old 6BQ5.
For a while, old Soviet military 6P14P- tubes were being touted as the affordable 6BQ5/EL84 equivalents. There are several variants of this tube. The 6P14P-EV, the 6P14P-ER and the 6P14P-K. Some are considered more rugged and longer lasting while others are reputed to deliver better sound quality. Some vendors sell some of versions of this Soviet military legend as 7189 equivalents. Be very careful. These tubes reportedly were overbuilt well beyond design specs required by the Soviet military. Still, the Soviet military specs sheets show B+ max of 250 volts. East German, Czech, Yugoslav and Polish production of these tubes from the communist era are also out there.
Hope that helps you.