If it's about cost, then "complete" kits don't seem to be the way to go for anythin' other then SLA setups (which I wouldn't reco) - as far as I've seen all of the kits that include a lithium type battery pack charge significantly more for that pack then any of the better ebike battery retailers would for a pack of equal or greater capacity... sometimes far more.
The complete kit sites aren't all necessarily a rip-off, but you'll pay for the questionable convenience of an all-in-one order, often the kit retailers rarely know or provide the level of detail needed to know exactly what the battery might be, which makes it difficult to know if you're gettin' your pennies worth. At best you're paying extra for some groovy case which is nice n'all... unless there's not much and/or lies inside it lol
If your friend is really interested in keeping costs down to provide a more economical choice in ebikes for the college crowd, then sourcing a better retailer for just the batteries is defo the way to go - some of them also offer cases & rack mounts for that aesthetic appeal, bmsbattery.com & pingbattery.com are a coupla examples of better battery retailers.
Hubs/motors are common & even the cheap ones seem to work really well, it's the batteries that mean so much and can't really be scrimped on if it's about cost
and customer satisfaction.... for ease of ordering the complete kits are good for the first-time builders as everythings matched & ready to go, but that's a very minor consideration for anyone that's built one before I figure
