Whether you pay $200 or $300, the initial cost of a string trimmer isn't the main cost of running it. Over an expected life of about 3 years and 1500 hours, the trimmer will drink about 300 gallons of gas ($750+), 6 gallons of 2-cycle oil ($120+), use up about 27 lbs of line (9 3 lbs spools or about $180), and wear out at 6+ bump head buttons (if $10 each, that's $60). Plus tax. Plus repair parts (although it makes more economic sense just to run them for 2-3 years, and then throw them away when they break--the parts are too expensive--and forget about paying someone to fix them).
The $200 trimmer will cost you $1310, or $437 per year to run.
The $300 trimmer will cost you $1410, or $470 per year to run.
Personally, I pick the trimmer I prefer the ergonomics and balance of. I'm currently on the 3rd year of my 2nd Stihl FS100RX (about $270).