$120 each for non-dimmable, $180 each for dimmable. You'd want two for a 75.
"Best" and "cheapest" seldom go together in this hobby.
As far as entry level cost, halides are probably the cheapest if you DIY. They do cost the most to run in terms of electricity. The bulb cost is probably the lowest. (Expect around $50 per bulb.)
T5HO are next. You can DIY these as well. As far as an "out of the box plug & play" solution, these are probably cheapest. They are middle of the road in terms of energy usage, but more expensive in terms of bulb cost. (Expect $20 per bulb. Normally 4 or more bulbs per fixture.)
LEDs are the new kids on the block with the highest up front cost, but they are getting progressively cheaper. There are some companies that sell DIY kits as well. There is no bulb replacement cost (or at least there shouldn't be) as the fixtures should last 10 years or more. They are definitely the most energy efficient. Some fixtures you can replace individual LEDs if they go out, some you can't. I'm not an electronics soldiering wiz, but I believe that is kind of what you're looking at as far as repair.
Remembering that watts are a measure of energy used, the fixture you found on Craigslist is going to use 516 watts (300 on halides, 216 on T5s). You could probably use an all halide system using two 250 watt bulbs for 500 watts. I use a six bulb T5HO system on my 75 using 324 watts. Two of the Taotronics will use 240 watts.
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