If you are trying to keep costs down, buying small plants is probably cheaper 
It just means you have less choice about what you plant, which isn’t a factor to me, as I plant them for my wife who wants me to cook with them (Big Beef works great and is very productive). I may do that this year for tomatoes- they are less than $1 each for the small ones and I don’t plant more than about 10. The cheap small ones are way better than I used to grow, and are probably similar to what I grow now with the light.
Now potatoes are another story, as they are for me 
. I ordered a bunch of interesting varities from Main Potato Lady last year and there are a few I really liked. #1 was Magic Molly, a purple fleshed potato with a rich nutty flavor. My #2 was All Blue, which as you can imagine, is All Blue (inside and out, though there is a bit of white mixed on the inside). Pic here
The most interesting one from a horticultural standpoint (as opposed to culinary) was Papa Cacho, a Peruvian landrace variety. They grow very strongly and seem resistant to anything nature throws at them. Mine grew 3’+ high, then were knocked over by a heavy rain storm, at which point they started growing up another couple feet. They lasted for a very long time, and I got quite a bit of production out of them. I dug 16 pounds of them in September (and probably 2-3 pounds earlier in the year), from just a 7-8’ row. They make crazy shapes and look almost like a sweet potato. The one problem- taste. I eat most of my potatoes baked and they tend to be dry, starchy, and plain that way. I finally hit on a way to use them, where they are pretty good- potato pancakes (shred potatoes, mix with eggs and fry on pan). Pic here
Now, if you want to keep spending down, Stop and Shop organic golden potatoes are pretty good. And you can plant quite a few from a 5 pound bag. Another option would be to get fingerlings at a place like Whole Foods or another specialty grocer. French Fingerlings are pretty good. I also had a purple potato called Fiesta which were very good (I only saw them once 
). Recently, I got some very good ones called Crimson Goldfinger (from Cinacia Organics, which also sells French Fingerlings, and Russian Banana). CG are very tasty, with dark yellow flesh, but most of them are very small. I may save and grow a few of the small ones, to see if I can size them up better.
I also saved a lot of potato seeds (not seed potatoes- they all got eaten) from Magic Molly and All Blue. I’m going to try to grow out a bunch of seedlings. I’ve never done this before, so I’m not sure how it will play out, but it’s worth a try to see what I can get. I’ll probably start the seedlings indoors, maybe sometime in the next month or so.