Asterisk is free and a simple FXO/FXS adapter with 1 port like you need won't cost $400. Try looking for linksys SPA3102 for example and an example config. You don't need to buy expensive multi-ports internal cards.
Why don't you go SIP and need no adaptor, only the internet conection your BOX delivers?
With Sip, you will receive the call for free if you have an online number. You can open a pre-paid account so if you only receive calls, you won't pay anything for the calls and you will use your credits only if you place calls. A lot of providers have this option. As an example, I use callcentric not free for online numbers but a few providers have this option free (I used voxalot for a long time before but it is closed now).
The caller may have to pay for a regular phone call. you can use sip services like sipbroker to be reachable from multiple sip providers for free for the caller if he uses one of the associated sip providers and paying only for a local regular calls on most big cities around the world for free. There are many sip providers that can supply you with a phone number almost where you want (most for a fee) so the callers pay only for local calls (similar to skype online numbers).