Costs vary from firm to firm but come out of the agree monthly payment, so are not an extra burden to worry about.
Most these days are calculated on a percentage of the monies paid in, typically 15%, but some charge 20 - 23%. As well as this there will be a fee for setting up the arrangement, around £1500 - £2000, seems to be the norm, and disbursements.
Variation meetings during the IVA will add £300 - 500 a time.
A few firms, I believe, still cost out on a time spent basis (hourly rate) --- which is wholly unpredictable at the outset.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014 http://foggy.blogs.iva.co.uk