The questions is simple.
Do you still need to pay a haggle price for services if you call a creature you can control?
Depends on what you're asking for, I think.
Spell sez:
Calling Creatures: This use of the spell has a material cost of 10,000 gp in rare incense and offerings. This cost is in addition to any cost that must be paid to the called creatures.
If you choose to call a kind of creature instead of a known individual, you may call either a single creature or several creatures. In either case, their total HD cannot exceed twice your caster level. In the case of a single creature, you can control it if its HD does not exceed your caster level. A creature with more HD than your caster level can't be controlled. Deities and unique beings cannot be controlled in any event. An uncontrolled being acts as it pleases, making the calling of such creatures rather dangerous. An uncontrolled being may return to its home plane at any time.
If you choose to exact a longer or more involved form of service from a called creature, you must offer some fair trade in return for that service. The service exacted must be reasonable with respect to the promised favor or reward; see the lesser planar ally spell for appropriate rewards. Some creatures may want their payment in “livestock” rather than in coin, which could involve complications. Immediately upon completion of the service, the being is transported to your vicinity, and you must then and there turn over the promised reward.
Gate is one of those spells that just scream for DM adjudication. As a DM, I'd probably have it depend a lot on what you wanted, and what you were summoning. If you, a 17th level wizard, use gate to summon up a horde of lantern archons -- probably no problem. If you use it to call a 17 HD marilith, she'll probably be wanting to talk with you. Seriously.
The stronger the enemy, the more it looks like a high-stakes negotiation. Even if you can control them, you don't want them to be your enemy after they return home.
If you want a for instance I'd be calling a planetar angel. But I don't think it'd matter if I can control it. Gate doesn't end like a summon spell so it's never not under your control at that point. You can just order it to not return home, work against you, etc.
Not pissing off some superior being to it would be a good reason to pay something for taking away their subordinate. But from a rules perspective I don't see why payment is necessary.
And should I order its service to be "serve me?"
I command them to not do that. Now what? I guess the broader question is what is the nature of that control then?
The nature of the control is that the creature can't do exactly what it wants, and it can't return home until its service is complete.
However, the spell quite clearly says "If you choose to exact a longer or more involved form of service from a called creature, you must offer some fair trade in return for that service. The service exacted must be reasonable with respect to the promised favor or reward."
In my opinion, there's no control until that bargaining is complete, assuming a longer service is what you desire.
It also says that if you fail to deliver on that promise, you will (at least) have to serve the creature or its master, or you may be hunted down by it or its kin. So it's probably not a good idea to try to circumvent the payment for rendered services.
I read it as you can control it for the duration of the spell, which is rounds per level, by only paying the 10000gp. If you want to control it for longer than that the "If you choose to exact a longer or more involved form of service from a called creature, you must offer some fair trade in return for that service..." line comes into play and you must bargain with it.