Grossly overpriced (relative to their assumed manufacturing cost) accessories are nothing new or unique to DJI. Look at car parts, cables, wine, restaurant/fast food, coffee, vehicles, most electronics, diamonds, prescription glasses, cosmetics, clothing, mattresses, pharmaceuticals, internet/cable plans, cell phone plans/overage charges, hotel rooms, and millions of other goods and services we all purchase often and use daily - markup is
insane. Many day-to-day goods and services have markup well into the thousands of percent. I used to work for a hardware store and let me tell you, 100-200% markup was on the very low end of the markup scale, and the highest markup was always on the tiny stuff. I dated a girl who was the manager of a high-end hotel, she told me it costs the hotel $15 per room per day on a $500/nt room. DJI's goal is to make as much money as possible like any other business, and they need to cover off R&D costs as well. Also I am guessing the prop guards on a Mavic Pro are not high volume sellers as I suspect most people are not using these indoors, which if I am correct would also make them cost more. At around 2% of the cost total cost of the drone, that honestly doesn't sound too bad for OEM prop guards. If you're upset with DJI for charging $29 for plastic prop guards, a quick look at all the things around your house will probably make you even more upset if you knew the profit margins

I actually find it fascinating.
$15 and 10-15 days is not unreasonable for shipping from China (assuming that is where they are coming from and you're in North America). Try living in Canada - getting anything shipped here from the USA is often way worse than that for a fraction of the distance. I wanted to buy a $200 backpack the other day and shipping was $60 USD + unknown boarder/brokerage fees +1.32 exchange rate. Polar Pro charges $35 USD for 4-day shipping to Canada on what amounts to a tiny envelope with a few grams of filters in it and $12.99 USD if I want to wait 2 weeks and risk having it held up at the boarder - slower than DJI's China shipping to travel a fraction of the distance on the same continent. Even getting a small package shipped one Province over is $25+ almost at a minimum unless you can fit it in a stamped envelope or want to wait 1-2 weeks without tracking or insurance, in which case it's about $5 cheaper.
Honestly $15 doesn't sound unreasonable for something to be transferred across ~12,000KM, multiple boarders, and hand delivered by a human to a specific address. Pretty amazing when you think about it actually. There is a minimum cost to do something like that, and shipping cost is based primarily on size/weight, so the fact that what you're shipping is inexpensive is irrelevant even though the total cost of it all may very well affect your decision to purchase.
Please don't take any of this the wrong way, as I definitely feel your pain, but when you break it all down it isn't always as unreasonable as it may seem

(I work in Marketing for a living). Like others have said though, it probably won't be long until a third party undercuts DJI by half or so and has them available locally.