Can you determine the percentage of leads that turn into paying customers? In addition, can you determine the overall worth of each customer? Ie. we have 200 current customers in the month of May and they have brought in a total of 20,000 pounds in revenue, that means that each client is worth 1000 pounds.
I know this is a rudimentary example, but if you can determine:
1- what a client is worth to you
2- How many leads become actual customers
Figure that out and you'll know if you are overspending for a lead.
Let's say you convert 20% of your leads to paying customers, then in the example given in your initial question, you will have paid 704 pounds for 4 paying customers. That is 176 pounds per customer acquired. Of course these numbers all hinge on you making sure credit is given to the appropriate marketing channel.