You can avoid SBC, but you’ve got to pay somebody to have “some kinda land line service” that technically includes DID and gateway. Depending on usage, you can subscribe to a VoIP provider that charges a small monthly fee (~$5) and a per minute fee for each minute of each outgoing call (Skype like) - I assume you are in the U.S. or Canada. Most people should be fine with this. If you just can’t live without an “unlimited” plan, you don’t need Asterisk for the total cost would be much more than going with Vonage or Lingo or iTalkBB. The extra flexibility of running your own Asterisk may not justify the extra cost. (In fact, iTalkBB allows you to pay $5/month and pay each incoming and outgoing minute, with free hardware.)
Now that you know a “subscription” is unavoidable, hardwarewise you’ll need either one or more ATA (and your old phone(s)), or VoIP hard phone(s). Either just below $100 each. That’s about all.