Seeing as I have absolutely 0 experience with this I'm not sure what to expect. I know they will often save you money in the long run, but I want to know the normal upfront fees a small business can expect to pay.
It really depends how big your small business is. What the turnover is and how many transactions you process a month will all affect it.
Are you looking for bookkeeping services? Do you want them to file your VAT return and PAYE? Do you want just the year end processed? How much work you do in terms of accurate bookkeeping and posting of invoices/purchases to the correct nominal code will affect how much work an accountant has to do at year end too.
Shop around and get at least three quotes. If you use something like Xero, the accountant can log in at the same time as you and carry on their work. If you use Sage, chances are you'll need to make a copy of the data file and send it over to them. This means you wont be able to work on your Sage file until they return it. Which is very antiquated.