Glorantha: I'd say runes cost 100 gp apiece. Runes are common enough that finding a rune is no problem, but finding the exact one you want may be difficult. Perhaps it's like finding a card for Magic - The Gathering. If you want a specific one and it is rare, you may be paying more. Spare runes of "the commons" would be quite common.
Runequest Modern: For this setting, it must be possible for PCs to make their own runes. The hard part is finding out how to do it, and a rune must be made with at least 1/4 ounce of gold for each magnitude of spell that the rune is designed to handle. This puts an effective price on a rune, especially a good one, but finding one is going to be pretty much next to impossible.
Iron Kingdoms: For this setting, I've decided that runes are essentially free. If you lose yours, you can make a new one with a difficult Runecasting check, but you can try once a day. The trick is finding someone to teach you the Runecasting skill for the rune that you want. I think this restriction should also be applied to the other settings (although runecasters in Runequest Modern should have some way to be self-taught).
To avoid too much mischief from extra runes floating around, perhaps Gloranthan characters should have to find a teacher to teach them how to integrate a rune they haven't integrated before.
As an aside, I don't like the way the rules use money as a balancing factor for magic. In many cases, I've found this does not make sense (e.g. for divine magic, did Jesus Christ say "If you have 5,000 gold pieces and the faith of a mustard seed then you can move a mountain"?) or is not balanced because either the PCs have too much money, and can buy any spell they darn well please, or don't have enough money, so that wizards and other spellcasters can not afford to do their work.
One option is to use hero points. If it costs, say, five hero points to integrate a rune, that will limit how many runes a PC can integrate, although there is still the matter of learning the spells. I'd use Intelligence to limit how many spells a character could know, and perhaps hero points to learn a spell.