１００ドルだったら買うって英語でなんて言うの？

You state 'final' price, usually after already going for negotiations..

To let the person know that highest price that you are willing to pay, you can say, "my highest price is $100." You can also say, "I'm not willing to pay more than $100."

This means that you want the price to be reduced to $100.

"I will buy it for $100." means you are offering them $100 for the product.

"I will buy it if it costs no more then $100." expresses to them that you will not pay more than $100 for it.

All the phrases can be used and are clearly indicating that you are willing to pay only up to $100.

Tom: I have a new watch for sale.
Me: How much does it cost?
Tom: $135
Me: I can offer you $100 for that watch.
Tom: Lets make it $120
Me: I can't afford to pay more than $100.
Tom: Okay, $110 ?
Me: $100 is the maximum amount and my final offer.
Tom: Okay, you can have it for $100.

1. If you wish to buy at $100, then your opening offer should be a little lower when you start negotiating.
2. Your budget is the amount of money you have to spend. If it is a cash negotiation, you may like to show the $100 you have as showing the money is often an incentive to the seller.
3. Here you state your maximum offer. Stating your maximum offer should be done towards the end of your negotiations.
   "I can't sell for less than $120."
   "Sorry, $100 is my maximum offer."

During the negotiation try to use any of these sentences and try to push the seller to agree on it using some words like "intend" and others.