if you are new to arbitrage know this:
 if it was possible someone with more experience would have been doing it already! so the difference wouldn't have existed in first place.

now lets look at the orderbooks.
if you sell ~1BTC the price will drop $10
and if you sell ~2BTC the price will drop $25

and with your 20K deposit assumption you get 8.4BTC
and selling that means price will drop $50

apart from withdrawal fee you will be paying trading fees too.
$20,000 * 0.25% = $50 * 2 = $100
i don't know how much the fiat deposit in bitstamp and withdrawal from cex.io will cost

you need at least $20,100 + as much those two fees cost.
now figure out how much the price should be for you to sell!
(20,100+ double fees) / 8.4