According to my DB the lowest price is $289.99 for Eachine ET8 Gas Double Valve Engine Model STEM (in Banggood). Just check coupons out (with price alert and history). What is best price?
Eachine ET8 Horizontal Hit and Miss Complete Gas Adjustable Speed Double Valve Engine Model STEM Upgrade Engine Toys
Latest coupon code price check (UTC time):
Banggood ($289.99)
Banggood warehouse is located in China. I have no active coupons for you right now. Low/high price? Hmmm, price is higher than average (which is $288.97 right now), so better set price alert down below…
Link: Banggood product page
Price: $289.99
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k and https://couponsfromchina.com/short/5cd
Old coupons (from the oldest to the newest): BG818bae, BG589b2e, BGMARCHSALE, BGAUGUST8, BGCAPITALOS8, BGCOUPERT8, BGBFDAY8, BGNEWYEAR8, BGSPRING8OFF, BGHONEYSP, BGESJY8
(there’s a little possibility that old coupon will work)
Aliexpress
There’re some similar (or not) items on Aliexpress
Click here to search by yourself. Here are coupons that you can use.
Price history and price alert
Price history (for all/most countries) for Eachine ET8 Gas Double Valve Engine Model STEM 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)).