I just started pvp a couple of months ago, on a couple of new characters, and I ended up going with purple weapons in the front, blue ones in the back. Mk xii polarized disruptors on my tac (cheapest at the time), and purple mk xi stuff on my sci. Turrets and torps, I tended to go a bit cheaper, depending on availability. Fitted out ships on both for about 5-6 mil ec each total, I think. [Acc]x2 is way cheaper than [acc]x3, and in some cases it's more interesting to go [acc][crith]x2 anyway (but there's plenty of threads about that around here)...
That worked out pretty well for me, I must say. Hoping to get access to some fleet stuff anytime now, but I do fine in pug pvp. Still running my blue mk xi consoles too, I must admit.
Often, a weapon of higher grade with the wrong mods will be inferior to a lower grade weapons with the correct mods. Mk 12 [acc]x3 will cost you millions of ec, but there are alternatives. Do not buy a epic ship weapons because it is cheap. Scout out the best mods for you. [crth/crtx/acc] are all safe bets if stacked.
Also, since [acc]x3 is so hard to get you can consider other stratagies to work around the others players speed and defense. Try to slow them down with a tractor beam and blast them with a volly of [crth] cannons.
The only thing you loose by having a lower mark grade is a bit of dps.
Each mark increases the damage of the weapon by 10% of the base damage. Everything that effects weapon damage is additive and goes off the base so you will get the same exact numeric increase from a tac console on a weapon weather it is mk 1 or mk 12.
Damage % buffs from abilities, such as APA, weapon power, and GDF do apply to all damage boosts.
For reference sake a beam array has a base damage of 100 and DHCs of 174 IIRC.
TLDR: Difference is relatively minor between mk 10 and mk 12. Best 'bang for the buck' weapons though are still fleet weapons even if they do only have a single 'acc' mod thanks to the ultra rare boost and 4 mods.