Race-winning quads that cost under $150? Yes.

This is another black friday budget built, with some clearance items, but can actually be copied quite affordably.
$128 is what I actually have tied up in this putting it into the air, and the end result is the fastest race quad I have from 50+ efforts.

The 'downside' is still motor efficiency, on 4S with 650mAh packs the race fligh time is only ~80s with the GF3052's, and the heavier 850mAh packs add a lot of weight but only increase full speed flight time by 15% or so, although I feel like a rockstar flying it that way because it's sticks to plastic in turns and straights.

Frame is the BetaFPV X-Knight 3" frame - chose this because replaceable arms are a huge thing for racing, and I was getting tired of writing off uniplate frames on 3" quads. If the HobbyCool asteroid comes back in stock, I'll buy more of those to use up 20x20 stacks, but practically the weight savings here running AIO boards are why this nearly identical powertrain to my other fast 3" builds make this an even faster quad.
Motors came from RDQ on Closeout sale - $9 motors are hard to say no to, although somebody looking to build a 6S version of this build would be happier paying more and running 2800KV.
AIO The JHEMCU 35A V2 with BBL - because it's the right price, and lightweight. Nothing fancy, if it burns it's a $38 unit, and what I'd expect to pay for a budget stack with the same capability. I've also added a 25V 220uF capacitor to it, because it's $0.20 and helps.
FPV Gear is another budget set of choices. The Caddx Ant Nano is among my favorite cameras on a budget, as it is a PAL/NTSC camera and hilariously cheap (under $13 ) and produces a good image for racing.
The VTX is the PandaRC $10 unit from Pyrodrone (buy them 2x at a time). For 25mW use racing, these work fine, the Tramp control is a bit finnicky, but does work. The antenna pictures is a cheap one, I've actually switched back to a dipole for weight.
Props are Gemfan 3052 Flash, which are just stupid fast, although they cost efficiency. I also like the Azure and Racekraft props in the same approximate size, as those are a touch smoother, but don't result in drastically different flight time, just a bit less top speed. I do need to try the TM3140's to see if that helps.
Batteries in this case are all CNHL units or on-sale Turnigy Graphene packs... because price. They see very hard lives (see fully discharging batteries in under two minutes), so no sense paying more than $7/pack if that's what they experience. That price is also why I build these as 4S quads, instead of 6S.

With more AIO boards coming, this concept is worth exploring

Part of me wants to see what an unlimited budget version of this quad could do, using the iFlight 6S 45A F7 AIO board, and running the XING 1408's as a 5S 700mAh quad, and keeping this frame setup. Also, the cost to run this setup is remarkably low, as the parts are cheap, batteries are cheap, and it doesn't require a big charging setup to keep packs topped up.

Overall, this is so fast in race conditions that I do question why I run 5" quads in many cases.