This topic comes up now-and-again. There are a few things to consider. At one end is a professionally monitored system using cellular. You usually need to use the modem they specify (which costs ~$150 - $200) plus monitoring. I have Next Alarm which runs about $250 a year. You can also set it up to alert you if the cellular connection goes down. Here that has only occurred once in 3 years for maybe an hour at 3am in the morning. (I'm assuming it was cell maintenance.) So this is very reliable. Offsetting that cost is maybe a discount on your insurance. Lets say $60.
Yes expensive, but it depends how much your house is worth to you I guess. The key issue if you "self monitor" is how will that work? I don't so much worry about a burglar, because a very loud siren is probably all you need, but more so with fire. So your away somewhere and you get a call that says fire. What do you do? Drive home? Call a neighbor? Call the fire department? Most people will try to "check it out" first and that wastes time. For me at least, not to have to worry about that is WORTH the extra cost. But really its up to you.
The C3 is rather old, and no longer supported, but all the wireless operators sell some type of wired phone to wireless converter, so you can connect wired phones in your house to cellular. The HAI acts like a wired phone so it should work fine with those. You might be able to find these on eBay also. For T-Mobile, as an example, its called LineLink and costs $30.
If you do go that route, make sure you plan step-by-step what you (or a family member) will do for each type of alarm. Then practice it at a given schedule. (With the monitored method, it runs a test weekly.) You will likely be panicked then, so make sure you practice and have this all planned out.
Your going to pay $10/month for added cell line, so that is $120/year. Monitoring is like $250-$60 discount = $190/year. So your saving maybe $70/year. Is it really worth it? Its up to you.