When you order a big ol' box o' resistors, You'll find they come in standard sets.http://www-k.ext.ti.com/SRVS/Data/ti/KnowledgeBases/analog/document/faqs/res_e.htm
that links explains it pretty well.
I ordered an E12 set a few years ago, in carbon film, with 50 pieces in each value, which worked out to like 2500 pieces for $50 or about .02 cents each.
I decided after it went thin here and there, that I preferred metal film better and ordered an E24 set, again 50 pieces per value and spent 1.2 cents each and it only cost about $90
When most of the vintage amps were designed and built, the industry standard was E24.
The E24 covers more than 90% of the stuff I need, you can order 100 of a specific value for less than $5 not including shipping