How to value this acquisition

Company A outsourced its services to Company B Company A paid B fees = $Y to provide these services, which Company A in turn provided to its clients and charged them $Z. Every time a service was provided Company A’s name would be in the letterhead. Now Company B decides to buy Company A’s client base and therefore Company B will be able to charge the clients $Z. For the Valuation: Should I value my acquisition based on the additional fees Company B will receive by buying the client base from Company A or the total revenues Company B will receive. (Even though Company B already received part of the fees it will receive now from Company A) Hope this makes sense. Thanks

Total Revenue. All the monies $Z and $Y should be with B. While the services are with the Clients. A is now out of the picture.