Find a low Hp, an old F3 is cheap, lots of spare parts and it is a very forgiving chassis, it absorbs mistake much better than a newer 600.
Cost will depend on a couple of things, how fast you are and hw much you crash, lucky they are not dependant on each other
Safety gear is most important spend more than you can afford
tires are the 2nd most important, In the begining you won't know when change them. Don't depend on the wear bars, the tires might heat cycle out before they are worn If you feel like you are losing traction then change them, how long they last will depend on how much you abuse them, either with the gas or with bad suspension setting. I burnt thru a brand new rear tire in 2 session (not 2 trackday) because of bad suspension settings. I also turned my fastest lap on tires that had 2 1/2 days of trackdays on them, they were very worn, but still gripped great because of my suspension adjustments.
Bring every tool that works on your bike and some that don't. You can borrow tools at the track.
You can spend a ton of money going to trackdays, remember that its a learning experience, so start slow, slower than you ride on the street and the take baby steps.
you want to gain speed thru skill not fear