$100/mo isn't outrageous. Being able to train 5 days a week is great, some places limit you to 2 or 3 days, and then only on specific days.
As long as you aren't being nickel-and-dimed on everything, and the training is age-appropriate, you should be fine.
But you mentioned two things that I don't like: one is that your instructor's son is a national winner. Who cares? That does not mean you will be a winner, and it doesn't even mean you'll compete there - even if you are that good.
The other thing is the WTF black belt test costs and WTF membership. You need to know that WTF and Kukkiwon are two different, but related organizations. WTF is concerned only with competition (and by extension, Olympics). Kukkiwon concerns only with black belt and instructor certification. IOC bylaws stipulate that the Olympic liaison (WTF, and similarly for other sports) not be affiliated with a particular country. So WTF (the Olympic liaison) is not a Korean entity, and Kukkiwon is a Korean government organization charged with certifying instructors and dan certs, which have nothing to do with the Olympics.
I don't know what the fees are to be part of WTF. But there are no direct fees to be part of Kukkiwon, though instructors do need to be certified with Kukkiwon in order to be able to award Kukkiwon dan certifications. I don't know what it costs to certify an instructor, but a 1st dan black belt is usually around $150. It is typical to charge a person applying for black belt for that charge, rather than amortize that rate over their membership fees, because that would make their membership fees appear higher compared to other schools - so it is purely a marketing concept to delay charging this fee. Most people in our world expect the black belt test to incur fees of several hundred dollars - another common marketing trick, which is designed to get you to train there for several years, put in a lot of blood, sweat, and tears, and then have a huge fanfare at the end: a self-fulfilling cycle.
As a result, I highly doubt your black belt tests are $80. Minimum would be $150 just to cover Kukkiwon. I've seen rates vary from $200 all the way up to $1000, as someone here on Y!A recently reported. Is it a scam? That depends. A place that produces top fighters that win national tournaments or gets their competitors into the Olympics will charge the higher end of that spectrum - and their monthly fees will also be higher. But, their facilities and trainers will be much higher quality as well.
Just because a place charges high rates and has a lot of trophies in the window doesn't mean you're gonna get those trophies or even compete with the top fighters. More details need to be considered.
Having said that, $100/month is reasonable, $70/belt is a bit high. And $80/black belt means either that the remaining $70 for that belt is amortized in your regular fees, or you are mistaken about the rates, or, you aren't certified through Kukkiwon (another common tactic instructors use to keep their rates low).
EDIT: whether you like it or not, places where you train do have expenses. The place needs water and heat, rent must be paid, taxes must be paid, and - usually - the instructor needs to buy liability insurance. Other expenses include instructor training (would you want to train with an instructor who stopped training?), and this can include his/her own memberships, testing fees, and travel. Also, some places like to host seminars and tournaments, and your membership contributes towards financing that.
Now, another aspect of your instructor's business is the competition down the block. For your instructor to attract and keep students, monthly fees need to be kept as low as possible because that's what people use to compare between schools. The incidentals like testing fees, uniforms, etc are often forgotten about until several months into training. Other schools bilk their younger students by creating weapons classes (taekwondo does not have weapons), leadership corps, and black belt clubs. These are money vacuums some schools use to offset the low monthly fees (and some are not really that low anyway - the biggest mcdojos are often the most expensive!)