Obviously the cost would depend on how expensive electricity is in your area. I'm not an expert, but since no one else replied, here's what I think:

Two 4-bulb T5HO (high-output) fixtures would cost about $20/month to run 12 hours per day... and provide about 13.5 watts per square foot of grow bed.

Four 4-bulb T5 (standard output) fixtures would cost about $16/month to run 12 hours per day... and provide about 14 watts per square foot of grow bed. These would provide better coverage (covers your whole grow bed) and are cheaper to buy.

That level (~15 watts per square foot of grow bed area) seems to be about the minimum amount of light for lettuce & greens & herbs to grow reliably.

If you could afford to run four 4-bulb T5HO fixtures then you're up in the 25+ watts per square foot range that tomatoes and peppers would prefer... for about $40/month in electricity.

A single 400w HPS or MH fixture only gets you 12.5 watts per square foot of grow area... and not very good "coverage" since all the light is right under the single fixture.

(Edited: more realistic electricity costs.)