Imagine you have a "Time-Machine": so you could go back in time to when Google Adwords was first launched. A time when you could buy ads for keywords in your sector for just pennies per click. How much would you spend? When clicks that might cost £20+ each today, might have cost less than 10p each back then. If you could go back, how much would you spend?
Your Customer Vending Machine
If your ad-sets were working and you were getting a return on your investment (i.e. you could spend £1 and get £2 back), then the answer should be that you would keep scaling up the ad spend. Like loading coins into a vending machine that spits out customer sales or leads on demand. There should be no limit on your ad spend under these conditions.
Yet did you hold back at that time - even though it was working? Being careful not to invest too much, just in case...
It's interesting to hear Gary Vaynerchuk (Mr. Internet Marketing Visionary) comment that one of his biggest regrets is that he did not go "all in" on Google Adwords when he was building his father's wine business in the early 2000's. Although he helped build the business to $60m, he laments that by fully utilising Adwords, he could have scaled it to $250m!
Yep, even Gary Vaynerchuk held back.
So if you find something that works: where you can load £1 in and get £2 back, don't get hamstrung by arbitrary marketing budgets. You should be able to scale this up. Keep going. You have found a market inefficiency. It will never be as "cheap" as it is right now.
Find and then milk your customer vending machine whilst you can!