You dont have to get a solicitor to do everything. I went to see a solicitor for one appointment to find out what I had to do and he filled in the initial form. I then did everything else myself, but you dont even have to have that one appointment. You can download the forms, print them off and do everything yourself.
It cost me £150 for the solicitors fee, £400 for the court fees (I believe they may have gone up a bit since then, but not much more) plus photocopying and postage. You have to get someone to sign a form to say that they have lost capacity and if you get the GP to do this they will charge, but I got mums SW to do it and she didnt.
At the end of it all there are further expenses as you have to buy a bond (a sort of insurance in case you disappear with the money!) and have to pay an annual fee to the Office of Public Guardians for overseeing the finances. Its not thousands though.
Once the deputyship is granted, some of the costs should be recoverable from the assets of the person you have deputyship for, I think. So you have to fund them upfront but won't be out of pocket in the long run.
Seems a bit expensive, so if you want to use a solicitor I'd shop around.
But it's always worth having a look at the forms first. That's what I did and they're not particularly complicated and no legal knowledge is required to complete an application. It's just pretty basic information about the person and their assets really, and my thinking was that I'd be telling a solicitor everything like that anyway, so why not just do the writing bit myself.
But it's an individual decision.
And yes, all the expenses to do with the application and then supervision fees and bonds are claimed back from the assets of the person you're applying for.