When it comes to the best budget gravel bikes, foregoing carbon fibre in favour of lower-cost steel or aluminium is a good way to save a good amount of money. Carbon options may be available, but at lower price points may be best avoided as manufacturers will need to make compromises to keep the costs down. That will probably mean the spec is worse, and it will also usually be made with a lower quality of carbon fibre and in our opinion good alloy trumps bad carbon.
The majority of the lower-priced gravel bikes will be made from aluminium. Aluminium technology is constantly evolving, and despite being a stiffer material, frame designers are able to now build aluminium frames with increasing levels of compliance and comfort. Meanwhile, steel is well-established as a forgiving yet durable material. It is generally a little heavier than alloy, but the ride quality it delivers garners a dedicated following. If you are touring you will also generally stand a better chance of getting steel repaired in remote locations.
The answer to this will depend on how you define 'budget'. If you can stretch to a carbon fibre frame from a reputable brand with good components, then you'll undoubtedly be happy with your choice, but the answer to this question lies in your response to the next question.