Ubercart

i like uc_recurring and auth.net

I think you only have two options: One is what UberChic mentioned; uc_recurring and authorize.net; and the other is lm_paypal (paypal subscriptions)
http://drupal.org/project/lm_paypal

uc_recurring is a module for interfacing with multiple gateways (it works with more then just Auth.net). If you want to use another gateway that is not currently supported its reasonably straight forward to add the functionality to your gateway to support uc_recurring.

But I think the question was about the cost of payment options, not modules support? To answer that question, it will depend on where you are located as most payment gateways only work in the country they are located (e.g. Auth.net is USA only).

There are a couple of options like Paypal and 2Checkout that work in most countries, both of these are cheap and have recurring options. 2Checkout does not have any real integration option with its recurring payments though, making it basically useless for site subscriptions where you need to keep track if payments are being made.

Paypal is cheap (free) to setup, but can be more costly then other option in terms of the fees it charges. Paypal is also limited in functionality then a gateway like Auth.net CIM which provides a lot more flexibility in its features.

For example using Auth.net CIM allows you to manually alter the recurring fees meaning you can do things like give a free month to a member. So going the cheapest option can be a false economy if your Business could be more profitable with the option to do things like offer competitions for free subscriptions and better handle complaints.

I don't use Auth.net with uc_recurring i use an Australian gateway securepay, but this gateway offers a similar set of features as Auth.net CIM.