The problem occurs when you need a certain part, search for months and eventually make do with something that's not quite right. Then the part on question, the one you need, turns up for the bargain price of $65 so you buy it. Then the same part, that you no longer need, turns up for $25 and you can't resist a bargain. Then it turns up for $12...
If you buy all of them you can average your cost.
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1973 Schwinn Super Sport
1986 Schwinn Peloton
1976 Raleigh Super Course Mk II(for wife)
1983 Gitane Super Corsa
1991 Trek 750 Multitrack