I considered several options from the DIY Harbor Freight/Northern Tool kayaks to Malone (Build It Yourself) to the Triton Watercraft aluminum welded trailers.
It kind of depends on your usage. If I just had a single kayak, I probably would have just kept using my truck bed or build off a used or cheaper frame (Harbor Freight/Northern Tool). Those convertible trailers are usually $500 new from those places and are either galvanized steel or aluminum, but the lights and wheels aren't generally rated to be submersible so you'll have to change those.
For me, I usually have 2 fishing kayaks (70+ lbs each) and occasionally a 3rd or 4th! So I needed something that could carry a bit more, but I also wanted something low enough that I can just back up into the water when possible.
The DIY route provided the greatest flexibility in adding on more supports/expansion and price, but required more work and I couldn't really lower it to my liking. Triton was the most expensive but had the highest weight capacity and has an aluminum frame (great for saltwater), but lacked expansion options. I did look at the Yakima as well, but the weight capacity was too low for my use-case.
Malone MicroSport Height is 30" from ground
Malone MicroSport LowBed Height is 22" from ground
HF Trailer is 25.5" from ground (no cross bars, so adding cross bars would increase this #)
Triton Personal Watercraft trailers Height is about 22" - 25" depending on models
In the end, for my usage, I went with the Malone trailer. I debated between MicroSport and MegaSport, but I really didn't need the additional capacity or storage of the MegaSport....though if garage space wasn't an issue, I'd probably opt for it.
Mine is a Malone MicroSport with the 2nd Tier Kit and Retractable Tongue upgrade. I'm mostly using the retractable tongue for winter storage in the garage so the family don't trip over it and kill themselves LOL... Similarly, if I went with the MegaSport, I'd probably look into an 3rd party fold-away tongue kit. In the future, I'm looking to add some 8" PVC on the 2nd Tier to carry some rods.
If you're using it for saltwater, need the weight capacity, or looking for a ready-to-go package, I'd recommend looking at the Triton trailers. It's honestly not that much more expensive than a Malone trailer. If you're focused on price, you should be able to get a used or build off a Harbor Freight/Northern Tool trailer and have it ready to go for $300 - $700.