Influencers potentially help you reach the target customer you are looking for – they can give you ‘voice’. First thing to get right (1) identify your target customer (2) work out where they sit, where they are – then think about ‘how do I reach them’ – and influencers might be one of these strategies. What I would do however is see if you can do some trials first to see if your assumptions are right.
In terms of ‘pricing’ – one test you can do is approach Socialites or We are Tenzing and ask them for a proposal to support your campaign to get a price – this will help you work out ‘how much it could cost’ for one off posts or a series of posts.
PS another hack is ‘ask’ a friend who knows an influencer to do it for free!!