It's credits - coins are a different thing in the mobile game.
If you enjoy it you have two more choices:
1) Play for free, that is, play with the free equipment and accept their weaknesses.
Take your time to build up some "free" credits by spending a lot of time for this.
2) Invest some in your enjoyment! Think of other pastimes you spend money for, days at the beach, nights out with friends, gear, cigarets and fancy food that you buy for yourself. So, why not spend money on the game? A tenner per week or per month isn't much compared to other activities(?)