This is almost certainly an artifact of me only playing SP, but I almost always fill the front line with all/nearly all mercs unless I have 200k+ manpower. It's not more cost effective, but it saves the trouble of managing attrition almost completely and leads to a neat trick. (I do keep one organic stack, usually.)
When the enemy is beaten, I can take all the merc infantry (usually 4x32) and combine them into a huge siege assault stack. Fun fact: if you have (5x fort level) worth of infantry and a 5/5 or better general, you can assault forts with essentially no losses. Sieging France down takes about a year, regardless of fort level.