I'm new to drones and want to create a DIY build for fun. Will the parts below work well together? Is there anything I'm missing? Is there any way to get everything below ~$400? If necessary, I'm fine with using my phone as a display instead of the goggles. Also is GetFPV expensive? Is buying from Banggood cheaper? What about hobbyking? Is there an easy way to attach a GoPro Hero 8 for cinematic shots?
List looks pretty good. If going 4S batteries (what I still stick with) then 2400kv option on the motors is one to go with. Only thing i think is missing maybe is the receiver for the controller. You can get a standalone video receiver that picks up the vid signal from the air and decodes most of the higher end goggles have swappable receiver module bay but the thing doing the decoding of vid signal is the receiver module itself the goggles are just something that takes analog vid in typically and draws the analog vid signal to screen but doesn't contain any hardware related to decoding the vid signal from the signals in the air (again that is the job of the video receiver module). There are 5.8GHz analog vid receivers that have HDMI or RCA video output lines too, problems with a screen are basically glare/visibility, having goggles masks everything else out so you are just seeing the vid signal (helps with being able to just see stuff but also with focus).
You can search around for quadcopter GoPro mounts or if have access to 3d printer can find STL files of models that you could print yourself (most of the ones you find online from retailers are also 3d printed since isn't worth making molds for such niche thing)
Regarding retailers getfpv does tend to be slightly premium priced but if in the US typically shipping is fast since believe they keep stock in warehouse in Florida vs having to go on a boat or whatever to get across the sea.
Skyzone CobraX Goggles from $150.00 to $329.99; TBS Source One V5 5" frame kit from $29.95 to $29.95; DJI V2 Goggles brand new from $500.00 to $500.00; DJI FPV Goggles v1 w/lipos and analog adapter from $0.00 to $0.00; Tiny trainer ELRS BNF from $0.00 to $0.00
^^ might be worth checking out for getting some deals on parts is mix of new/used things but can get a good discount on new or lightly used things from what I've seen (not shopped there a lot but if looking to save would use it more)
hobby-king is alright in terms of general RC products and if just getting like a quantity of batteries or bigger motors or something can be worth going through them but is as far as I know all china based then so shipping times might be higher for most products depending on where you are.
Regarding banggood or ali or the like in general things are "cheap" there but is a "get what you pay for" situation from what I've experienced overall. Can be fine for somewhat throwaway stuff like props when first learning or getting slightly discounted deal on batteries for small quads or something, but generally isn't worth the wait in my experience and if disappointed in the product there's little to no course for action. Here is a list I had from a while back but again don't think there's anything major you missed there and the price is about in the right ballpark for pretty good quality components.
Sheet1 Part Type,Cost,Usage,Link Frame,50,Holds everything in place. Ideally want a rigid frame that won't break when moving at high speeds.,<a href="https://www.racedayquads.com/collections/iflight-xl5-frame-spare-parts/products/iflight-xl5-v4-true-x-5-frame">https://www.racedayquads.com/colle...
Some things to keep in mind too regarding cost the transmitter/controller and the goggles are typically not going to sustain serious damage (fumbles of controller or goggles aside), basically if you take good care of them they will generally last a pretty long time (years) a quad on the other hand is likely to at least break a prop if not an arm or something more major during flight (especially when learning, suggest simulator time with velocidrone or lift-off games). All that to say if you spend more on the controller and goggles can see it as bit of investment in the hobby (if unsure how long will be interested then keep it a light investment but if all in then worth saving up for good transmitter and goggles).