"This isn't close to what a decent horseman has to pay for a very squishy and huge (read arrows will hit) horse, and even more when the horse is easier to hit and a little more resistant."
Once again, people are yet to realize the intentions towards cavalry from the mod creator. It was supposed to be a "prestige class" due to the simple fact that it is the biggest powerhouse in this mod/game and the biggest tide-turner. A decent cav/lancer p.e. can easily achieve ridiculous amounts of kills even using the "squishy and huge horse" you kindly mention, (take Leed, or Tommy as an example). And the reason behind the increased cost of the mounts prices it's only a measure to counter the huge numbers of cavalry constantly at the game, making it simply unplayable, specially with the number of deser/plains maps on rotation now.
You can imagine 30+ players with a 1 hit kill ability every few seconds and the ability of knocking down opponents while dealing damage as they please.
Also add the fact that it's easier with medium/high ping players to hit the rider than the bugged mount, cause it's immense the amount of arrows that pass by that amazing animal or that every cav rider can bump hit anyone trying to block (yes, sadly even lancers can do it, even if the lance has already passed the target).
Even so this measure wasn't enough, since market allows people to make easily lots of money by either honest trades or scamming like a boss.
Look at all the noob cav we see lately, how many kills do they make ? Not that much.
Besides, it is correct that cav is the class with the best killing potential, but when you look at it closely, they can't really choose their kills. And that makes a lot of difference. Killing a lvl 3 peasant or killing that-2h-celebrity-that-killed-your-whole-team-last-round counts as 1 kill. There are very few things that will determine wether someone will probably die to cav or not : cav awareness, teamplay and weapon choice.
The peasant has usually no defense and will get lanced or bumped to death cheaply. One kill.
The uber-elite-2h has cav awareness, otherwise he wouldn't be that much elite. He has a good anti-cav defense weapon (jumpswings, GS lolstabs). And sometimes plays nearby teammates, sometimes not, but since he has a good weapon and awareness, the team support isn't needed. To kill that guy, a cav has to wait for an opening in the awareness, otherwise he will get lolstabbed. He also has to wait for the teammates (especially polearm users) to die or go somewhere else. Moreover, attacking when the 2h is occupied fighting inf is a gamble, as the risk of killing/bumping/hitting a teammate is very high, especially with "tornado fighters". And finally, he is forced to use the couched lance and all its hazards since a decent armor is enough to protect the 2h from a onehit lance poke (and hitting that 2h twice will be even more difficult). If the cav manages to kill that beast somehow, the holy scoreboard says the same. One kill.
No wonder those very good shiedless infantry are usually killed by ranged, as all the other classes have much more trouble doing it.
What I'm trying to say is that while cav do a lot of kills, most of them aren't valuable at all. I can safely assume that 70% of cav kills are unaware people. The rest are fail jumpswings, ranged missing the last-second-headshot, other cav or footmen outnumbered by cav. Those people with poor cav awareness usually aren't good in what they do, cav awareness is a sign of battle mode experience (thus general fighting experience as well).
Ranged, on the other side of the spectrum, can choose their targets in a much larger pool. The differences between the peasant and the uber-2h from an archer point of view are much lesser than from a cav pov (unlike cav in the majority of cases, ranged don't die when they miss). Yet one will turn the tide of the battle, and the other will not.
That's why I don't think cav do turn the tide of battles more than any other classes. Elite cavs turn the tide of battles just as elite inf or elite ranged. Average cav ? Nope. They die trying to charge a lone archer (it takes a decent horseman to kill the logically weakest possible aware target) or doing other things that seem realistic but aren't effective. It's only when they realise they are disguised ninjas on horses they start making kills (yet not the "useful" kills).
tl:dr Archers don't make lots of kills but all their arrows (hitting or not) are useful for the team. The K/D ratio doesn't mean anything compared to the W/L ratio.