I just started a campaign as Bulgaria in High, and my first unit in the queue was an emissary. I insist on this - first bribe Constantinople, especially because of the amount of income it offers (and that's with or without a governor); then bribe Greece. The valour your emissary gets from the first bribe should lower the cost for Sgouros to about 3244 or so (and by now, with taxes on high in Bulgaria and Constantinope, you'll be gaining around 2000 florins per turn, so that will help temper the bleeding of your treasury). I found that bribing Constantinople cost about 3000 florins or so, give or take a few hundred (but not much), and gives you the important valour point to lower the cost of bribing Sgouros.