If you already know what happiness is and how it effects the NPC, feel free to skip this part.

In the new 1.4 update, NPCs now have individual preferences about living near other NPCs or what biome they live in which in turn affects their prices. The lowest prices they can get down to are 75% of the original price when they are happy. The highest prices they can get up to are 150% of the original price. So, this system benefits players who treat their NPCs with care and not don't just cramp them all into one small box. You will need to raise the happiness of any NPC Vendor before they will sell you Pylons for the biome they are currently in. You can have a max of 2 NPCs within 25 blocks and a max of 4 NPCs within 120 blocks, otherwise it will be considered "over crowded" and raise prices for all NPCs affected. Get more of a discount for putting them with NPCs they love vs just like. If you put them with NPCs they dislike or hate, then prices will go up. This doesn't affect the Travling or Skeleton Merchant; they use different mechanics which I'll cover below.

Forest

• Guide + Merchant - Near/in your main base in the forest
  
• Zoologist + Golfer - Anywhere at least 25 blocks away from the Guide and Merchant so you don't get negative happiness

Snow

Prior to moving the Tax Collector (or Santa) you will need to move Mechanic and Cyborg to the snow biome and buy the pylon.
• Santa - Likes the snow but hates Tax Collector so make sure he is far enough away
  
• Tax Collector + Any NPC Pet - Likes the snow, I'm not sure if happiness affects how much/fast he makes money needs testing but will most likely move him once I get Santa. Need 2 NPCs to use the Pylon and pets count as an NPC to use Pylons.

Desert

• Steampunk + Cyborg - Best prices for Steampunk, she loves Cyborg and Desert and Cyborg likes Steampunk
  
• Gun Dealer + Nurse - Getting best price from Gun Dealer

Jungle

• Witch Doctor + Dryad - They like the jungle and each other
  
• Painter + Clothier - Painter likes the jungle and needed a spot to put the Clothier (I don't value him very high if you can't tell he is free to roam)

Ocean

• Pirate + Angler - Best prices for Pirate for buying items to bolster your defenses for invasions
  
• Stylist + Dye Trader - Best prices for Stylist, she loves the Dye Trader and Ocean

Mushroom Surface

• Truffle + Any NPC Pet - Truffle will spawn on Mushroom surface, but you will need to create it manually first while in hardmode. Need 2 NPCs to use the Pylon and pets count as an NPC to use Pylons. You will need to move another NPC (Dryad for example) with Truffle to buy the Pylon first then you can move the Dryad out and move the Pet in

Hallow

• Wizard + Party Girl - They both like Hallow and each other

Underground

• Demolitionist + Tavernkeep - Love each other and likes underground, good to have the pylon here for gem tree farm
  
• Goblin Tinkerer + Mechanic - Best prices for reforging with the Goblin Tinkerer

Traveling Merchant

His prices are highest the the closer he is to the initial spawn. The further away he is from the initial spawn the lower his prices are. He has a chance to randomly spawn each day next to 2 NPC's. So be sure to have town in Ocean biome and what ever biome is next to your dungeon and hope he spawns there for the lowest prices.

Skeleton Merchant

His prices and item he sells are affected by the phase of moon which I will show the price change below. During the daytime his prices will go up 10% and during night time there will be no effect to prices. Note that he spawns randomly underground and will despawn as soon as he is off screen.

Here is an example of the layout I will be using. 8 x 8 | 11 x 8 | 8 x 8 (counting outer walls)
This leaves room for you to add some chests or decorations if you want around the pylon.