Having trouble saving up money? Here's the fastest way I could find: if you're after the trophy that forces you to "restore the Thieves' Guild to it's former glory" you surely have done a ton of random quests forn Delvin and Vex. You could of course keep doing missions (keep in mind, Delvin's quest about chinging numbers are by far the easiest and fastest) until you reach 100,000 but it's nice to mix it up.
Simply put, as you "restore the guild's glory" you'll unlock fences, vendors who'll buy anything from you even if it's stolen, whatever category it is, and have at least 4000 gold on them. So hoard as much as you can and visit them regurarly, selling pretty much averything you might not want. Keep in mind that enchanted armor and weapons, dragon materials and jewelry are among the most valued items and fetch good prices. People mock speech, but it can slightly speed up the process.
Keep on selling your stuff to the specific vendors from the guild and make sure you sell EVERYTHING that you don't need and you can get to that sum pretty soon. It also helps if you try to save as many spetims as you can as if it was your real-life money.