If you’re spending money and it’s profitable, but you’re restricted by budget (ie there’s the opportunity to increase your budget and spend more), then I would always recommend if you can and have the cashflow (or more importantly, the business capacity to cope with more orders), then by all means increase your budget. Spend as much as you can afford, as long as it remains profitable.
‘Enough’ is a relative term: if you’re getting ‘enough’ business and you’re happy with the level your business is at right now, then you’re spending enough.
If you want to grow your business and your campaigns are profitable then you’re not spending ‘enough’.
It’s a decision you have to make based on your business and based on the profitability of your current campaigns.