The bottom bracket (also called BB)  is that cartrige hidden between the cranks and is part of the “engine” of a bike, so is just a matter of quality rather than aesthetics..

Better don’t be a penny-pincher on a good BB, but I would never spend more than $50 if you use the bike for as a commuter.

Before buying the BB you should know which crankest you would use, because the clearance between the chainring and the frame may vary a lot and you have to know the exact length of the bb. They come in different size from 103mm to 117mm and usually the manufacturer of your crankset can suggest the correct bb length.

Brand to search for:

My favorites are Tange and Miche, because they are good and cheap. Phil Wood is awesome but for my need is too expensive.

I don’t like Sugino’s entry level bbs as they come with a plastic threaded cap.. I bet it works and is lightweight but I want it METAL!

Before you buy:

If you are going to buy a used/vintage bottom braket be aware of the thread because it has to be of the same nationality of the frame: for instance. if you have a japanese or english frame you need an ISO thread; if you have an old french go with french, italian goes with italian. In some rare cases you might find interchangeability but I dont see the reason to do it.

You have to know the chainring size you are going to use otherwise you risk to have the chainring touching the frame. In the worse case you have already bought everything you might can tight more the left side so the right so it can be more exposed and distant allowing more clearance for the cranks, nut I’m talking in case of 2/3 mm maximum.

How to install:

Use grease o an anti-seize compound (ParkTools) and the relative tool, ex. Miche is different from Sugino and other brands, so remember to get the right one, check the ParkTool website or the brand of your BB to see which tool you need (the choice is between 3 or 4 model so is not hard to find the right one). Usually the tool cost starts form $5/used.