I had a different tactic for collecting $$$.
The sooner you grab a coin, the more it's worth.
So flying over the enemies, and collecting the coins as soon as possible, gives you more cash than finishing a wave successfully...
My only weapons were lightning guns, rocket launchers and bombs. The rest of the grid was coin magnets. That way, I could just focus on picking up money for further upgrades. When it came time to fight the boss at wave 27, I swapped-out most of the magnets for more bomb chargers, ensuring I could crack-off about four of those to go with the lightning and rockets. The boss went down easily with that arrangement.
Not too difficult a game at all, but still a fun time-killer.
1 full row of lazers, 4 rocket modules & 4 lightning modules. Put a double column of magnets below your ship all the way to the bottom. You can get this by level ~15 and gets 100% kills and coins until past level 30, also kills the boss, barely. Game only took 20-30 mins to complete, but much fun!