I can't foresee paying that kind of money for an app either. But, I'm also not a professional in that field.
I do own machines that do nothing but apply paste to the back of wallpaper and cost between $3,000 and $4,000...
We also use specialized tools that apply joint compound to new sheetrock walls. A company called Ames Tools makes them and you cannot buy them. They only rent them. It costs good money to do so.
Like with any specialized tool, the market pays what it can bear.
Those pasting machines btw, will allow an individual to earn $100 or more in additional income... per day. In the space of 1-2 months it will have paid for itself. Worth every penny of $3k. With a crew of 5 men hanging what would be the equivalent of 500,000 rolls of wallpaper in a new hotel, that machine can pay for itself in as little as 2-3 days vs applying paste by hand. The next 5 years is pure profit off of it.
edit: It is also tools like these that folks invent that helps to keep prices down. As an example, the labor price for installing wallpaper in many areas of the country, is today, within 10-20% of the price I was charging 30-40 years ago. There are the master craftsmen who can demand double and triple that amount, but we're talking less than 5% of the folks in this trade today.