Business Insider just ran a great profile on Martin Greenfield--one of the largest tailors in the USA--who produces fantastic suits for brands like Brooks Brothers, Band of Outsiders, and a number of made-to-meuisasure programs from his factory in Brooklyn. See it here.
So what makes a suit worth $2,500? While quality fabric from renowned English or Italian mills can run into the thousands, details like hand-set sleeves and pad-stitched lapels are both labor intensive and require a high degree of craftsmanship. You can learn more about the anatomy of a suit here.
If you're looking for a made-to-measure suit at a more manageable price point, outfits like Kent Wang and Black Lapel produce well-made suits that also look great with a Benchmark Briefcase.
While the internet has made it easy to get a relatively high quality custom suit, remember that each manufacturer's "house cut" (aka style of the suit) is different, which includes aspects like gorge (height of the lapel), shoulder padding/style, and button stance. These details are generally consistent in all suits by the same tailor.
Make sure you do your research before taking the plunge. Whenever you meet a tailor in person, ask to see some examples of their work to see if their style meshes with yours. If you're going the online route, take a look at their gallery or look book.