NeuroVoider

20,000 scrap isn't a lot if you just keep playing through the levels. The trick is to only upgrade what's necessary. If you feel like you're getting close to 20,000, take the chance to complete the next level without upgrading. For me, I just played normally and upgraded only orange items. With fully upgraded orange items in every slot, I was able to save up 70,000 scrap at the end of the game on Arcade difficulty.