Nicotine River is a reputable vendor (from what I have read), and know of several (at least) ECF participants who use them regularly.
I am not a huge fan of kits personally; but that is a matter of personal choice. I am a cheap bugger, and am willing to spend the additional time to find the best value, rather than pay a mark-up for the convenience of one stop shopping. Other find their time extremely valuable, and are happy to pay more to save time. And, as
@bombastinator mentioned, "how much" of the kit's items you will actually use.
The $50 kit I see on NR's website doesn't seem out of line. It looks like you would be getting somewhere between $19-$34 worth of flavor concentrates (depending on flavor/brand selected). And the balance would be applied towards: the liter of base (PG and/or VG), 150mL of 48mg/mL nic, and misc. mixing hardware. If it all gets used, and if shipping isn't much, it seems like an alright price (and certainly better than others I have looked at in the past).
If, after trying your hand at DIY, and
if you decide you like it,
then you might want to look at using various vendors for specific needs (e.g. concentrates from vendor "A," bases from vendor "B," and nic from vendor "C"). Buying in larger quantities, and cheaper price points,
can more than offset the price of paying multiple shipping charges.
You are smart to start off slow, and buy small quantities, until you know that this is something that you will want to pursue. Keeping it simple is always a wise practice.