It typically comes down to experience, with other factors added in, such as geographic location, time taken and value of the work to the client (branding for a large company is more valuable to a sole-trader and should be billed as such). There's also the issue of client budget. I can charge an amount for an identity but if it's outside of the client's price-range then either they either need to accept they'll be getting a fast, less-polished design, simplify their requirements, or find another designer who's cheaper.
For basic identity design I'd personally be charging between £300 and £500 as a minimum. That's for quite a basic package of a logo with minor revisions, and probably some basic branding elements included such as colour schemes, patterns, typography, maybe even some stationery if budget permits. That price is also based off of my personal circumstances such as business over-heads, demand, experience, etc.