O2 UK has posted the pricing for the new iPhone 3GS on its website, they are offering pricing for both Pay Monthly and Pay as you Go customers.

For Pay Montly customers who are looking to have a 24 month contract the iPhone 3GS 16GB will cost £87 on the £35 per month tariff and will be free on all the others. The iPhone 3GS 32GB will cost at most £175 or as little as £0.
iPhone 3GS O2 Pricing matrix
For those Pay Monthly customers who wish to only have an 18 month contract the iPhone 3GS 16GB will start at £185 and work its way down to free, the 32GB model will be at most £274 down to £97 on a £73 per month tariff.

For customers wishing to pick up an iPhone 3GS on Pay as you Go, the 16GB model will cost £440.40 and the 32GB £538.30.

O2 will also be offering MMS and Internet tethering. An MMS message will cost four normal texts on Pay Monthly where are PAYG users will pay a hefty 25p.

O2 will only offer iPhone tethering with an applicable bolt on, a 3GB bolt on will cost £15 a month with a 10GB capped bolt on costing £29 per month.

The iPhone 3GS will be available in O2 stores, O2 online store, Apple retail stores and Carphone Warehouse stores on June 19.