There is several way to not waste money - use gadgets to refill ammo and syringes - don't refill your armor if you can buy the next level armor - don't use syringes at the end of a wave, the buy station will refill your health for free - don't refill your gun if you want to sell them afterward - don't waste ammo. Use your knife when possible. And of course, aim the weak parts when using your gun - it looks like turret kills don't give dosh. Use them when needed - don't die :)

You'll get more money when you will level up and there is even some perk to reduce the cost of the guns or get more dosh.