I use Airtel which works reasonably well. They have 3G in the major towns (excluding Koidu). I have friends that use Africell 3G and they report good results. I did look at using Africell for data, but they were not permitting modem use when I asked last year. I do not know whether Africell have 3G outside of Freetown.
Whoever you use, the fibre optic cable is not active yet, so do not expect the speed to be good during busy times.
Have a look at the Airtel web site for the costs. It says 15 units for 1MB. 1 unit equates to around $0.01.
In addition to what DaveS has said, I was in FNA over the Xmas period and not once did I go into an internet cafe. I did all my surfing via Airtel on my fone. This included gmail, gtalk, whatsapp, play store (I've an android fone JellyBean 4.2.1) and the web. Speeds were okay considering it was FNA.
Registered Linux User # 125653 (counter.li.org)
Now accepting personal mail for GMail invites.
The administrator has disabled public write access.