They are LF rather than HF, and unprotected rather than protected or armoured. And obviously they are armed with bow rather than being impact foot, swordsmen
They do need to be the same quality rating as the rest of the BG, however.
So they are costed by choosing the separate line in the list with the appropriate quality rating.
If you deploy them in the front line, then no they don't fight at all like legionaries!
Assuming you have them as a third line, as is the intention, they get to shoot ONLY in the impact phase, and only against mounted. (This is true of v1 - not totally sure it hasn't changed in v2). And they won't fight in melee at all.
Their benefit in a continuing melee is that a) they increase the size of the BG and make it a bit harder to inflict an extra -1 on a cohesion test for 1 hit per 3 bases. Although TBH I don't think I have seen that make a difference on more than a handful of occasions; and b) when you lose a legionary base, a LF ends up in the second rank to replace the base that fills in the hole in the front rank. That LF base does fight exactly as if it is a legionary - it contributes one dice (LF lose 1 dice per 2 in combat against mist opponents, so when you only have one LF dice to start with they don't lose any) at the same POA as the front rank base. A bit odd perhaps, but that's the game mechanics. When you lose a second base however, you do lose a dice now that you have 2 LF bases fighting.
So the general feeling is that they are only cost effective in a BG of relatively expensive troops, because the effect that 2 bases of support LF produces when added to what would otherwise be a 4 base BG of MF/HF is not too dissimilar to the effect that you would get if instead you added a single base of MF/HF (if you were allowed to do so).
Thus adding 5 pt LF bases in support of average armoured legionaries costing 10 pts is more or less break-even for cost-effectiveness. Adding 6 pt LF to superior armoured skilled swordsman legionaries at 14 pts looks like a distinctly good deal.
In V2, the -1 POA for shooting during impact is removed, so the appeal of LF archer support when you expect opponents to have a lot of shock mounted may be increased.