Sorry to sound like an Eeyore Straightandlevel80kt, but like a lot of things in professional flying, the cost is always more than you expect. You should also consider the total costs of being a flying instructor (renewals, medicals, travelling etc) as well as the cost of training. In my experience, the cost of training was substantial but does not get anywhere near the loss of income I have suffered from being a full-time flying instructor.
I have certainly been very lucky as an instructor to get decent work, but it has not been well paid nor close to home. Over the last two years my income from full-time FI work (I am unrestricted and an IMC instructor) has been less than £4k per year after deducting travelling and accommodation expenses. The average rate I have been paid is about £15 per hour or £10 with a daily retainer of £20. the best has been £20 per hour in a remote school that took most of my income in travelling and accommodation.
If you want to instruct weekends at your local club then this post is not relevant. If however this is a career in the making, then budget for a couple of years of cr@ppy money before moving on to either professional flying instruction with one of the big schools or airlines or examining. Living away from home, missing the family and being paid sh1t money is hard...especially if the weather is pants and you're not flying. The 'glamour' of flight soon wears off when you can't pay your bills.
Still...must get my fix...so I'll carry on.
ps. Someone on this forum once described flying instruction as 'a hobby that has got out of hand'! If you enjoy flying then just buy your own plane and spend the money enjoying yourself...it will be cheaper in the long run. If you have a masochistic streak...then go for the FI career. Crazy and fun yes, remunerative ? NO.