A T-1 is 1.544 mbit/sec. It is a digital service, by definition.
How many voice channels you can cram onto it is a matter of how you do it.
IF you're using it as your link to the telco:
You can treat it as an ISDN PRI , which gets you 23 B channels (voice or data at 64kbit/sec) plus 1 D channel (shared).
You can treat it as a "DS-1", which gives you 24 DS-0 (56kbit/sec) digital voice channels. Which can be converted to analog by a 'D-4' type "channel bank", or equivalent.
IF you're using it for a private point-to-point link, you can use various kinds of compression on the link -- as long as you have compatible equipment on both ends of the link -- you can get 24, 48,64, 96, 128, or even 192 voice circuits over one such link. The fidelity of the voice reproduction goes down as the number of 'lines' on the T-1 goes up. At 48 voice 'lines', quality is 'to the ear', virtually indistinguishable from a regular phone line.
'Cost' is whatever that whomever you're buying from says it is.
And things vary tremendously depending on where you are located. A difference of a few tens of kilometers can make a tremendous difference in cost. A difference of 'across the street' can make a50% difference in the cost of a 'bare' circuit (i.e. not including the 'value' of the service provided by 'whatever it is connected to'. (e.g., the actual telephone network services -- dial-tone, phone number, etc.)
In the U.S., ISDN PRI is frequently more expensive than a DS-1, the telco figures that they're providing extra/advanced services with PRI, so it should be worth more to the customer. And have priced it accordingly. But a few telcos do it differently -- wanting to encourage the high-tech use.
Either IDSN PRI, or DS-1 requires 'CPE' (equipment _at_ your location) to utilize it. This takes he form of a PBX with the appropriate kind of 'trunk' line card in it, and the corresponding software, *or* (for a DS-1 only) a 'channel bank' to convert things to POTS analog lines -- into which you can plug any kind of 'standard' telephone appliance.