It depends on what kind of duel you wish to undertake. The most expensive solo duel will set you back 1 Crystal and 25 gold. Squads will be 3 Crystal and 50 gold. And for warr, it will set you back 10 Crystal and 100 gold. In $ terms, that's relatively inexpensive. It's about $1 for the most costly squad duel. But that doesn't count gas fees. And that's just for one duel, which will be over quickly. You'll probably duel multiple times, so you're looking at more $$. And if you don't have a way to generate that money in-game, you'll need to pour more money into DFK constantly. And that's not just expensive; it's tedious and time-consuming.