The IP will assess your essential outgoings (utilities, food, rent / mortgage, car running costs etc) and will deduct the resulting figure from your total income. What is left will be your IVA payment, if sufficient.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014
depending on the IP they may have a minimum amount, for some it is as low as £50 - but I do know some set it as high as £150.
If one says you don't have enough surplus then try another - they may be willing to take on a client with a lower surplus.
Other than that it will simply be what you have left over after costs are deducted.
Just like to add it's worth talking to 2 or 3 professionals to discuss all your options. We have experts that are well respected (you can find their email addresses etc in the ask experts a question section) or you can contact some companies on www.iva.com (where you can check out what other people think of their experiences).
Always worth checking whether bankruptcy is a better option to an IVA.
Bankruptcy does seem an attractive options if you have no assets to protect although an IVA may also be possible. Get some free advice from an IP firm to see what is best.