I see that you have capitalized "pay". Last I checked, I had to pay for an ambulance. The ambulance is not more powerful than other medics. You can't even really quantify the medic cost for OKW, because they get it as a package deal. It's not mobile, but can be put in a good position, no delay on healing, a huge amount of armor and hp, and it's a shot blocker. It's a different cost for a different level of utility.
So ambulance is op, because it can heal more than 3 squads at a time? It doesn't even stay active long enough to fully heal badly damaged troops. The ambulance can only do 1 thing and has limits placed on it's operation, so having a decent amount of acceleration proportional to a real ambulance is not something that will make balance worse. 250/10 is not exactly cheap for something that is going to take up your pop cap and not fight. It's actually pretty close to the cost of half-tracks without the teching cost.
You pay
less.
OKW have a 70 munitions 3 pack of medkits to heal individual squads and the Medic truck. (Which cost 40 fuel to upgrade to, and the medics can be killed temporarily/have to walk to the units to heal them.)
Wehrmacht have a 60 munitions upgrade on a bunker, and can only pick EITHER reinforce or heal. They also have single use medkits as vet 1.
"pretty close to the cost of halftracks".
Halftracks are 30 fuel. And you can't fucking discount teching costs. The amount of time and resources it takes to get mobile reinforcement for other factions is much longer, and they can't heal their troops at the same time either. (Healing reduces bleed in future fights, reducing the amount you need to reinforce.)
Anyway I'm not saying ambulance is OP. I'm saying that it shouldn't be able to zip away easily if your army is pushed off because it's such a powerful defensive tool. Kind of like how if you're forced to retreat the enemy can destroy your bunker.