Just check coupons out (with price alert and history). What is the best price?
Output frequency is: 50Hz for 220V model /60Hz for 110V model, Weight: 0.6 kg, Peak Power: 4000W, Input voltage: DC 10.5V~15V, Output voltage: AC 110V /220V for your choice, Package Size: 6.4×4.4x 2.2 inch, Type: DC 12V to AC110V / DC 12V to AC 220V, (When the battery voltage is lower than 10.5V / the inverter is low voltage protection / when the battery voltage is higher than 15V / the inverter high voltage protection), Output Waveform: Modified sine wave, Rated Power: 300W, Model: SP4000, Product Size: 13.5*11.2*5.5cm, Name: Car Power Inverter 4000W
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.
Link: Banggood product page
1. ($35.69): BGCarEle4
2. ($39.47): BG00c906
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): BGHXM009, BG2020CAR, BGH0090901, BGCuponation10, BG10OFFAFF, BGwhielabel, 13f142, 00c906, ADMITAD2212, 2022ACTIESNL, BGCarEle1, MVC8SPRING, 8BGwbg2204, BGCarEle3, 9OFFSUMMER
(these codes were not working in latest coupon update, but you can check them out anyway)
Price history and price alert
Price history (for all/most countries) for 300W Car Power Inverter 12V-110/220V 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)).