See the Runes of Magic page describing how they accept Zeevex tokens here.  It is reproduced below for your convenience.  There is no one answer as to the exchange rate between Zeevex tokens and Runes of Magic diamonds, as diamonds are sold in packages and are subject to promotions as well.  For example, there was a 10% extra diamonds promotion this past weekend!

In general, the more diamonds you buy at once, the more diamonds you get for your money.   So it is better to buy all the diamonds you need at once early, rather than buying them in small amounts as you need them.

Don’t want to wait 30 days for gift Diamonds or Diamond trade? Use Runes of Magic prepaid cards!

Our payment system now accepts Zeevex Xtreme Online Game Cards. Purchase a Zeevex card or get one as a gift, and use Zeevex Tokens to buy Diamonds.

The cards are available at Blockbuster and FYE and soon will be available atToys R us, 7-Eleven and Rite Aid. Zeevex cards provide instant access to in-game credits, bypassing the traditional hold period for PayPal transactions and credit card payments.

See Diamond Trade page for more information. 

Zeevex Xtreme Online Game Cards come in increments of $10 and $20, depending on the retail store. The activated cards carry 1,000 or 2,000 Zeevex Tokens respectively, which can then be redeemed at the Zeevex's Redeem Your Card page.

The exchange rate is $10 = 1,000 Tokens. After you have redeemed tokens, all you need to do is visit our Diamond Recharge page, select a diamond package, choose Zeevex as the payment method and then you will be redirected to the Zeevex page.

Then simply verify the payment. Afterwards, you will get the diamonds shortly after both systems process the payment. Keep in mind this may take several minutes.