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The cost of the diving courses is very difficult to say. It can range widely from region to region, shop to shop & even instructor to instructor. Some course fees include book fees, some do not. Most (not all) courses require the students to have a minimum of equipment that typically includes your own mask fins, snorkel & boots. If you want to look at additional gear, I would suggest waiting until after your OW course. That may give you some ideas as to where you may want to take your diving. Different forms of diving may take different typ[es of equipment. If nothing else, you can start with a simple BC & regualtor set. The best thing you can do, is to interview & ask questions from the shop/ instructor. Get details on how the course is run & all fees involved. One thing I would be a bit leery of is any shop that "requires" you to purchase your gear only from them to take their course. If you choose to buy from them, that's fine, its your choice.
As for what courses to take, get through your Open Water Diver course first. Get some diving in, see where your interests take you. One course I would say that is an excellent compliment to OW, without getting into specific specialties is a Rescue Diver course (after getting some experience & comfort in your OW skills). Once you think you may have a certain interest, get with your instructor & discuss how to best attain your golas. Once again the cost can vary according to region, shop &/ or instructor.
There are a few branches of diving that you can investigate after your OW course. You may choose just to stay within the area of simple recreational diving (absolutely nothing wrong with that), you may even eventually look at technical diving, going into the proffessional ranks (Dive Master/ Instructor), commerical diving or possibly public safety diving. Know that the latter types of diving typically require quite a bit of experience to do them. The best thing you can do after you complete your open water diver course is to get out & dive, including diving with those at your level & those with more experience who may be able to help mentor you a bit.