The cost of topping up with crystals if you're paying with Euro and with card or paypal is 0,01€ per crystal. Ingame you get 2 cc per 1 crystal. So let's say you want 6000 cc, you can top up with 3000 crystals, which would be 30€.
On this site oldsch00l.lebanni.net/runecalcEN/ you can calculate the total cost in gold, cc, and gold dust depending on the level and quantity of runes you want to create.
Personally I lower the bar 1 step every time I create a rune (a little more gold dust and less cc used). This is a matter of personal preference though as it depends on how much real money you're willing to spend and how much ingame gold you're willing to spend.
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