Why debug probe vary so much in prices?
The price will be determined by a number of factors including but not limited to;
Development Costs,
Production Costs,
Market segment size,
Market share,
Target consumer revenue/size,
Competitor pricing,
Consumer Bias (more expensive=better).

All of which combine to influence the final price.
For example if the cost to develop is high and the market small then the unit cost will be high.
Or if the product has a high value to the consumer (eg saves them significant time) then the cost may be high.

Of course competition influences prices too as a smart manufacturer sets their price as high as the market will bear and then drops the price to undercut competitors effectively pricing them out of the market.

A dominant manufacturer may maintain their margin in the face of small competitors to discourage another large manufacturer too.

In your specific example the price variation is most likely a combination of features and sales figures with a dash of productivity.
The $99 probe has less features but suits smaller customers while the $1500 probe has more features and probably would not scare off a large corporate buyer if the features and quality matched the requirements.