I am not going to buy it, especially because I don't have money.
I will not buy it because I don't have any money.
The Immediate Future is used for a plan or intention.
The Simple Future is used for a decision made at the moment.
Here's an example. Two friends are choosing ice creams:
A: What flavor are you going to have?
B: I'm going to have vanilla. That's what I always have.
A: That's so boring. Why don't you have something different. The blueberry one is amazing. I'm going to have that.
B: Mmm, OK. I'll have that too."
We can use 'will' when we are very sure indeed about the future. We can use 'will' to contradict an idea, and to insist on the certainty of something.
>> I can do this, and I will do this.