In answer to your question – the service would not cost you anything. The service is actually paid for by the creditors. You would pay a contribution, say £400 a month (depending on your monthly income and expenditure) over five years which would give you a total lump sum of £24,000. If you opted to go for an IVA you would propose an IVA to your creditors and you would have a creditors meeting, where your creditors would vote on your IVA. At this meeting your creditors would decide what fees they felt were reasonable for your case to be managed over 5 years. Usually this comes to about £8000 total fees over five years. But you would not feel this expense as your creditors would give it to the IP out of the lump sum you were paying to them, so for example, out of your £24,000 the creditors would give the IP £8,000 and divide £16,000 between them. The only time you would have to pay directly would be if you proposed an IVA and were rejected. These costs are calculated on a time-cost basis.
Depending on your case, you may also be asked to pay a drafting fee for the professional work undertaken drafting your proposals. How much this is depends on your case, but is usually in the range of £500-£1000, which can be paid in installments.