Although there is no actual bonus for farming prestige, it only brings you up in the rankings, players will fear you and may not attack you when you are far higher in prestige than them.
The way prestige is calculated is by your town hall level, so if you have 10 cities with ten level 10 town halls you will get only a low number of prestige for your attacks. If you only have 1 city with a very low town hall you can get massive amounts of prestige for your hits.
True Prestige Farming
The true way of farming prestige is to build up a massive army from your 10 cities, then give away/abandon all of them and capture a lv2 NPC. Next you need to get your rally spot, workshop and academy up full. The idea behind it is you will have as low a town hall level as possible to allow you to upgrade the essential building, e.g. workshop for you gear and academy for you TECHS. Now you can start hitting high level NPCS and getting 100s of thousands of prestige a hit. You can build up millions in a few days. Many players do it before leaving the server to put them on top of the leader board for a while, and many alliances get their members to do it to keep them at the top.
Farming Prestige with High City Count
Even if you have 10 cities with all lv10 town halls, you can still maximise the amount of prestige you can get per hit. The less troops you send at a NPC and win the more prestige you get. So if you still want to farm prestige when you have multiple cities you can. The method is simple, you want to send as little warriors at a lv9 NPC as possible while taking as little losses as acceptable.
An example is this...
If you send 100,000 warriors at a LV9 NPC with Lv 100 hero with all gear set to Intel and a Lv 9 star set, your losses will be around 23,000 warriors and you will gain about 500 prestige ( if you have 10 cities all with Lv 10 town halls)
But if you send 70,000 warriors at a LV9 NPC with Lv 100 hero with all gear set to Intel and a Lv 9 star set, your losses will be around 25,000 warriors and you will gain about 1000 prestige ( if you have 10 cities all with Lv 10 town halls )
As you see for a few extra losses, you gain twice the amount of prestige. So if you hit a large number of NPC's you can maximise the amount of prestige you earn and gain 100s of thousands a week. You can also do it with lower level NPC's like 8s (send 60,000 warriors) and below but Lv 9s are the best in terms of amount of prestige earned.