ISDN is generally not cost-effective compared to other forms of broadband. And that's not taking into account the relative slowness (64 or 128 kbps) compared to other broadband.
You generally have 2 ways of connecting:
"Dial-up" ISDN. Just the ISDN BRI connection will probably cost you more than DSL. Then you need to add some charge per-B-channel, per-minute (last I checked it was 2 cents). That's 4 cents per-minute if you're running at 128kbps.
"Centrex ISDN". Means that you and your ISP are 2 branch offices of the same company, as far as the phone company is concerned. They will charge you per-mile, each month. When I checked into it, it was several hundred dollars.
ISDN
You generally have 2 ways of connecting: