i run a small tape-label (by the name of MMODEMM) – roughly every 2 months, we release tapeboxes which hold 5 cassettes à 5 minutes, each with one track from a different artist ... "cassingles" we call them. occasionally we release EPs..
the process isn't really complicated (which is the great things about tapes!), but there's no rule by thumb i'd say.
there's a variety of ways to print on tape... if you have your tapes dubbed, the service usually offers to do this for you. there are also different printing techniques if you want to go DIY – google will help you. last, there's the option to use stickers, of course. this is generally the simplest route to go in my opinion.
any printing store will do this for you. you'll find layouts online. if you want your jcards cut, you'll have to pay a few bucks more. depending on the strength of paper, it's very important to crease the jcards correctly, else the paper may break
often self-dubbing! the "pro-"route is to have multiple tape-decks of the same model, ideally with a syncing option. this way you can pop 10 cassettes in and press "rec" on one of the decks. we're often using TASCAMs (302), which work very well but need occasional maintenance. when self-dubbing, be sure to do a test-run and check that the levels are to your liking. this is something you'll want to play with and not necessarily utilise "by the book" – it's what makes tapes neat in my opinion also, be aware that you'll most probably loose high-ends and get a boost in the lows ... prepare your mix for this.
i'm from germany, so i probably can't help you on this.. although we often order from the UK (tapeline is a good spot!).
generally, there are two common types of cassettes – the ones with the brown (ferro) tape and the black (chrome) ones. black ones sound better – i'd use these.
depends on the route you go, but per se (esp. compared to vinyl) it's SUPER cheap! rather vague number, but you'll make a small batch (~100) for around 150€ i think.