There may well be others that are around the price of the kit Lencarta sells, however I don't think they'll be any cheaper. You tend to find the boom arms sold as kits, as the fitting on the top of the stand is a lot more robust than a simple lighting spigot - to handle the rotational force. They also have wider footprints than a light stand, and come with wheels - which make it harder to tip over as lateral forces (from someone knocking it) just pus it across the floor rather than pivoting it around a one of the feet. I have one that's identical to the Lencarta one
https://www.lencarta.com/studio-boom-arm-with-casters and it's held up well for years. I paid over a £100 for mine though from elsewhere. £84 is a good deal. The counterweight is sufficient for most jobs and I've had a Lencarta Superfast 600 (3kg) and a 120cm octa (~1kg) on the end. The counterweights are around 5kg I think, but as you tend to have more pole extended at the light end, it won't be perfectly balanced. This is ok as long as it's not too much out of whack and the you remember which is the heavy end. You do need to be very deliberate about handling a boom like this though -try to arrange the light so it hangs naturally in the position you want it rather than relying on the wing-bolt on the light to stop it rotating on the end of the arm. Note - the Lencarta boom arm (and many others) allows the spigot to attach to the arm in two ways - straight out, and at 90 degrees to give more options for hanging the light. Align the arm with one of the legs. Be careful when taking the light off as the counterweight will pull the whole thing over. Do as Chris does and add sand bags over the crossbars or wheels.
You'll have less problems with a Smartflash as they don't weigh as much, and an arm like the Lencarta one will hold that up just fine. For the best boom arm experience see below the Safari II - the light that brings balance to the force
The light side and the pack side balance out
I use the Safari on the boom (well this extension arm on a C-stand mostly tbh as it takes less room than a regular boom stand) most of the time for this reason - if I don't need an all Superfast setup for freezing motion.
The third way of supporting a light overhead which I now use almost all of the time for heavy lights is to use a background support (two background stands with 3m crossbar) and attach the light to the middle of the bar with a super-clamp. This is a much more stable and safe arrangement.