I'm working on a MonoTouch In-App Purchase sample, you can check out the code here:
https://github.com/conceptdev/xamarin-samples/tree/master/InAppPurchase
The sample contains two projects, one that demonstrates Consumable purchases and one that demonstrates NonConsumable purchases.
NOTE: the example does not yet demonstrate RECEIPT VERIFICATION, so you'll have to add this in yourself (why? see hack, fix).
Check out @redth's server-side code to help build your own receipt verification logic with ASP.NET:
https://github.com/Redth/APNS-Sharp/tree/master/JdSoft.Apple.AppStore
You might also consider this services like UrbanAirship or Beeblex (although I have not tried them, so can't) for receipt verification.
FYI my sample code is based in-part on @jtclancey's AppStore code here:
https://github.com/Clancey/ClanceyLib
Finally, there's a bit of set-up required for In-App Purchases (registering your bank details with Apple, setting up the products in the iOS Developer Portal, Provisioning your app correctly). These steps are the same for MonoTouch and Objective-C, so this Apple setup doco might help. You should also read Apple's In-App Purchase programming docs, for familiarity.