Don't be silly. You get off the plane, and have two choices, the limo (what I call it) for 150 baht, or the taxi. The limo, which is what you're referring to as the mini-bus, is 150 baht. As far as I know, those are the only two choices. You can call it a mini-bus, or a van, or whatever you want. A real "limo", as is used elsewhere, is something I've never seen.
There was also a price for a taxi, which I think was far more than 450 baht. This is at the counter right there at the airport. If I remember correctly, the prices are there on the signs, but there's never been a need to ask - it's one or the other. My memory says it was 500 baht, but since I don't take it, I never really paid that much attention to it.
If you know as much as I think you do, you must know that the van stops part way to Patong, to separate the passengers from Patong from those going elsewhere, and they *always* try to get you to change hotels to one that they think is better (and which I'm sure gets them a nice commission). If you didn't know that, try it sometime. They *always* ask where you're staying, and that's *always* followed up by an attempt to get you to change, sometimes even after you've told them that you've got everything all set.