The cost for buying a settlement depends on 3 factors: the size of the seller's empire (larger empires are more willing to give up small portions of land), the wealth of the settlement (would you ever give up Baktria for a pittance?), and the distance of the settlement from the capital (factions like to exchange a rebellious border province for some cold hard cash). Some rough estimates/actual amounts of the costs to buy settlements based on these 3 factors are listed below:

Numidia making its first purchase of Ikyoshim from Carthage: 7000 mnai

Ptolemaioi ending the Syrian Wars by buying Antiocheia and Damascos on turn 2: 30000 mnai

Buying one of Sauromatia's last 4 settlements to trigger their migration and enable Sauromatian Enclaves: 10000 mnai

Acquiring Sardeis and Ipsos from the Seleucids as Pergamon on turn 1: 13000 mnai

Buying any Seleucid settlement far away from the capital in the early-mid game: 7500-12500 mnai

Buying any one of any faction's last 2 non-capital settlements: 40000-??? mnai

Buying Garamantine: 12 mnai and a bag of Cheetos

Based on these estimates and facts you can expand economically should you not fancy triggering war or fighting siege battles. A noteworthy fact is that buying a settlement that seals off an enemy army besieging one of your formerly bordering settlements will ALWAYS force that army to retreat. Said army will also be "confused" for one turn, guaranteeing your a sliver of time to reinforce your newly-bought settlement. If you don't like expanding economically, you can still buy provinces with a clear conscience by selling those provinces on the same turn using another diplomat. You can Balkanize entire regions this way by divvying them up between competing powers, ensuring that the resulting chaos, war, and human suffering will keep your borders safe for a LONG time.