Is this possible?

It's not even close to 75% of your salary. It's just over half.

$90,000 is $7500 a month. She's asking for $4150. Half would be $3750.

Isn't your son covered by your health insurance under your employer? It shouldn't cost you anything. Ditto for her until you are divorced.

That lowers the bill to $3450, less than half.

You don't pay tax on alimony, she does. That increases your take home pay by about $600 a month.

Paying for the house sounds reasonable. You do want your child to have a roof over his head, don't you? You both bought the house based on your salary and she doesn't have one. She's not going to be able to buy another one.

You can't go back in time. She hasn't been working because you agreed as a couple to have a child. She gave up time in a career to raise your child which was a direct benefit to you. Now you need to accept that there are consequences for your decisions. 

Why wouldn't you want to pay for your child's college education? You're his father and this should be a given.

If the car is close to being paid off, it might be cheaper just to do it. If it's nearer the beginning you might be able to trade it in for a cheaper one.

Everyone loses in divorce. When you first look at it everyone reacts badly. On closer inspection it isn't as bad as it sounds. Still, it's a starting point, so you can negotiate. The car would be the place to start.

In my state she could go to court within 10 days and have you pay Post Separatoin support starting immediately. That could keep you in limbo for years and the courts only consider two things: how much does it cost for her to live and can you pay. They don't care if you have to camp out on a friend's couch.

I don't think she's being at all unreasonable. It's going to take her some time to get through the emotional stuff before she can even consider getting a job. Divorce is hard.

I think that seems very unreasonable and alot to ask for.  
As a divorced woman she needs to pay for her own living expenses as well as her own transportational needs.  She is asking to live like a married woman, but not be married. She cant have it both ways. 
However, you do need to pay child support for your child. The courts will usually set a certain percentage.  Then usually other agreements are settled if there are any unusual expenses above and beyond that.
One thing I often notice on these boards is people saying things like "she's been sitting around for 5 years as a SAHM" - well as a former SAHM, I can assure you she has not been 'sitting around'.  Being a SAHM is probably significantly more difficult than your job, and obviously she gets no respect or recognition from you for what she does. Perhaps she isnt really good at it.. even doing it badly is still work.  And that doesnt even figure in the fact that she's given up vital time to advance her career.  She just doesnt have the earning power you have now because she's just given up those years. 
You really need to take that into consideration. It's not just 'sitting around' - it's work.. and sadly there is no respect for it...

Don't sign anything and go consult with a lawyer.
 
I talked to one for 3 hours and it cost $150.00..........may be the smartest thing I did in the entire 11 year relationship with the stbx.

She has worked but not in the past 5 years. I've supported her as a 'stay at home mom' during that time though she hasn't held up her end.

I haven't signed anything. This was her attempt to scare me (which worked) but the more I thought about it the more illogical it seemed, so I wanted to check with others.

I make 90k and she's made 0 in five years. She likes to claim she helped support me through grad school but was still making 70k during that time..

Honestly I could probably prove her as mentally disabled based on her prescription history but technically, no..

Child is about to be 5 - married for almost 9..

How does 50/50 work? 2 weeks per month with me/2 with her? Sorry to be a dumbass but I don't know much about this - and her father is in the legal arena - so I'm disadvantaged..

If you signed and had it notorized it can hold up in court. I did the same thing only reasonable so that we did not have to have a mediator. We also did a divorce by default which means that we are divorced but he has 6 months to appeal with a lawyer any property settlements.

holy moly!

*cough* *gag* *choke*

She's dellusional. Even if she is an at home mom, she is dellusional.

Has she ever worked in her life? Or does she now? If she does work, how much more do you make than her?

Is she disabled at all?

How old is the child?

How long have you been married?

On custody, sorry, you aren't a weekend daddy, you are going to go for 50/50 joint physical and legal I'd assume, correct?

Answer the above questions and I can give you a bit more input.