An interesting blog post on the subject here:

http://goldengekko.blogspot.com/2010/09/what-are-costs-of-developing-mobile-app.html

NB. The numbers are pretty accurate in my opinion.

Update: Also of interest are some of the comments in this stackoverflow question. Some frank estimates of time by some of the developers for well known apps such as twitterific.

http://stackoverflow.com/questions/209170/how-much-does-it-cost-to-develop-an-iphone-application/3926493#3926493

If  so, then you can reduce your monthly fee by £15 by simply converting to the Simplicity 20 tariff on Pay As You Go.

At £20 per month for 600 mins + 1200 texts and with unlimited internet, free cloud/open zone wifi and visual voicemail – this is actually a better package than I had on my £35/month (18 month) iPhone contract.  Crucially, this is a one month rolling contract so you are not tied to 02 for another 12/18/24 months.

Also, don’t forget to unlock your iPhone from o2!
http://shop.o2.co.uk/update/unlockmyiphone.html