For my
Different Take on Imbuements, I priced the ability to
change between a default attack type and functioning as though it came from a specific source (sunlight, gold, etc) at +20% for Very Common, +40% for Common, +60% for Occasional, and +80% for Rare. Always counting as a given source is arguably worth +0%, but I could see cause to halve the above prices instead.
And, yes, I largely used Extra Damage Type as something of a base. Indeed, I believe the GURPS damage categories (imp, cut, pi, etc) all rate as Common, so you could use that as justification to reduce the above to +10%, +20%, +30%, and +40% (for switchable), or halve those (for static).