You should spend money on good content to support developers, but that doesn't mean you have to always pay full price. I mean, if there's a sale, shouldn't you take advantage of it? Of course, and here are all the best sales live in the Play Store.
Apps
CamScanner: $1.99 from $4.99
Sleep Cycle alarm clock: $0.24 from $0.99
Star Walk - Astronomy Guide: $0.99 from $2.99
Star Walk 2 - Night Sky Guide: $0.99 from $2.99
Runtastic PRO: $1.49 from $4.99
Cameringo+ Effects Camera: $0.10 from $2.99 (not for US)
Backdrops Pro: $0.99 from $1.99 (in-app)
Puffin Browser Pro: $0.99 from $3.99
WeatherPro: $0.10 from $2.99 (not for US)
Water Drink Reminder Pro: $0.99 from $1.99
Games
Cosmonautica: $3.49 from $5.99
har•mo•ny 2: $0.99 from $3.99
Lara Croft GO: $0.99 from $4.99
Hitman GO: $0.99 from $4.99
Minecraft: Story Mode: $2.49 from $4.99
Thrive Island - Survival: $0.99 from $2.99
Auro: $1.99 from $6.99
SRPG Legna Tactica: $0.99 from $7.99
The School : White Day: $4.99 from $7.69
Colin McRae Rally: $0.10 from $2.29 (not for US)
Last Horizon: $0.99 from $2.99
Size DOES Matter: $1.99 from $4.42
Oddworld: Munch's Oddysee: $0.99 from $2.99
Oddworld: Stranger's Wrath: $0.99 from $2.99
FINAL FANTASY VI: $11.00 from $15.99
OTTTD : Over The Top TD: $1.49 from $2.11
Templar Assault RPG Elite: $0.99 from $2.99
Dokuro: $0.99 from $1.99
Battlevoid: Harbinger: $0.99 from $3.99
Earn to Die: $0.10 from $1.49 (not for US)
Tsuro - The Game of the Path: $2.99 from $4.99