Just check coupons out (with price alert and history). Best price?
Product Weight: About 226g, Product Size: Transmitter is about 190*113*70mm /receiver is 122*18mm(length is included with wire) /bind wire is 50mm length, , Item Name: B4 Radio Transmitter with B4A/B4B RC Receiver, Appliction: RC Cars boats tanks vehicles models, Material: Plastic and electronic parts, Model: B4, Receiver: B4A (3+1CH)/B4B (4CH) (Optional), Brand Name: RBR/C, Color: Black
Aliexpress
There’re some similar (or not) items on Aliexpress
Click here to search by yourself. Here are coupons that you can use.
Latest coupon and price update (UTC):
Banggood
Banggood warehouse is located in China. I am tired of searching coupons, but I found one for you…
Link: Banggood product page
1. ($26.99): BGRADIORU
apply it in your cart after clicking “Proceed to checkout”. Click to copy code and open shop.
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k and https://couponsfromchina.com/short/5cd
Old coupons (from the oldest to the newest): BGe13500, BGRCParts8, BGJUNEGSG9
(there’s a little possibility that old coupon will work)
Price history and price alert
Price history (for all/most countries) for RBR/C B4 2.4GHz 4CH RC Transmitter with B4A/B4B is here.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood and Aliexpress (only if product name will be the same as on Banggood and I will post it on social media though, so it’s more like Banggood price alert)).