You have to be able to size up their market, their company, some idea of sales/volume/revenue/etc. Sometimes it's easy to ballpark, perhaps based on headcount (even assuming part time workers, you could probably ballpark $50k per person - a 50 person company would be grossing ~$3m/year just to stay open in the US, I'd think, but in most cases it would probably be 50-100% more than that, depending on industry, location and type of workforce).
Looking at a small company that's pulling in $500k, there's almost no way anything you do for them service-wise would be worth $300k to them (short of you taking over operations or similar). For a company pulling in $50m, there's almost certainly $300k of value you could bring to some department, doing probably the exact same work you might do for the $500k company.