Hi there. I also owed just over 40k when I entered my IVA, and my initial proposed montly payment amount was similar to yours. I too thought it seemed too good to be true, but when they called to say it had been accepted, everything changed right there and then.
As mentioned above, your creditors would rather have the guarantee of some return on what you owe them, with the prospect of that amount going up if your income increases etc. It's a daunting prospect, learning to live on what you will initially see as a tiny amount every month, but it can be done, and learning to live like that actually changes your mindset on spending.
I made the last payment into my IVA in March, and although I still haven't had my completion certificate (grrr), I've actually found that I still tend to live within the monthly budget that my IP set me, and the rest goes into savings.
Chin up, you won't belive the good nights sleep you get that first day your IVA is approved
