Is there a discount to Global Entry if you already have TSA Preferred?
(I assume you mean TSA PreCheck?)
Unfortunately, no there's no discount for Global Entry if you already have PreCheck, so you have to cough up the $100 GE application fee even if you already have PreCheck. It's why I think that in most cases (unless the traveler really does very few international trips) it makes sense to pay the $15 premium for GE, to get both GE and PreCheck in one.
But you know what's even better? Financially speaking, the best program to apply for is Nexus (US/Canadian description). For US$50, you get Global Entry benefits, expedited entry into Canada and use of trusted security lanes in both countries (PreCheck in the USA and CATSA Trusted Traveller lines in Canada) -- so more benefits for less cost.
In other words, it's basically getting the equivalent of PreCheck and Global Entry but for both Canada and the USA, and to top it off for half the cost of Global Entry.
The catch is that you have to interview with both US and Canadian border authorities at an enrollment center (in the same way that you have to do an interview with US CBP for Global Entry), and all the enrollment centers are either in major Canadian airports and cities or US locations right on the northern border -- think International Falls (MN), Pembina (ND), Blaine (WA), though there are centers in Detroit and Niagara Falls.
So if you're not already located close to the US/Canada border, it's probably not convenient for you, in which case I think Global Entry is then the best deal. (For Nexus, you also obviously have to be admissible to Canada and to satisfy both US and Canadian authorities, which might be a problem for some.)