Everything below is my own experience. Different companies have a different process and also take into consideration that this is with a UK bank rather than US, but here's the basic info:
We tried to get setup with Lloyds TSB Cardnet.
http://www.lloydstsbcardnet.com
They wanted £250 setup cost + an annual fee (yearly), 3% Base rate charge for CC transactions, You can't take AMEX over the net, it has to be done via phone (ie: You would need a terminal and a direct line to lloyds to then take the CC numbers and then ring up to get the card verified and charged, lengthy process and time consuming.) Amex is setup separate from the standard account, simply because you have to get Amex setup with Amex, rather than with Lloyds. Amex wanted roughly 2%-4% on top of lloyds 3% base rate..

(You can't setup with Amex until lloyds have your account setup up.)
Further to that if you wanted to deal in multiple currencies, (ie: Dealing with the US) you would need a separate account. So one account for UK to UK transactions, and then another for UK to US. This is so that you can take into consideration the currency conversion ect.. (Oh yeah, dealing in a different currency is subject to 0.5% increase in base rate charge.)
Dealing in multiple currencies means more cost as well, simply just taking into affect the exchange rate.
Before any of this is possible you need to submit a business plan, objectives and estimates of turnover and estimated profit levels. After that your account is subject to "Risk Assessment", if it fails that then no account.
Finally, if you don't use your account at least once a month they charge you a minimum or £30.
So in conclusion, having chatted with the Rep for quite a while, a merchant account (with lloyds anyway) is more for business to retail, rather than business to business. Wholesale deals (as they involve more risk than retail) will probably not get through the "Risk Assessment" due to the high volumes involved and rather risky process.
Any more questions that I haven't covered then I am more than happy to advise, do bear in mind it is a highly complex and frustrating process.