i had to do this just recently as well.
for most cheaper services, you are limited by 3 things:
ability to backup network drives
amount of space allowed
and number of machines.
keep in mind 150GB over your ISP will take days to backup. if he doesn't need the ease of use and availability an online backup solution offers, i would just recommend a few external drives, back them up and place a few different places/ safety deposit for instance.
with that being said, if you are going to pay for a service, get a good one. the best, most professional and best value i have found is with a company called SOS backup
$10/month. which is 2x compared to most $50/year services, but the space speed and quality make it worthwhile, definitely.