I found someone still have a bunch of 50W mercury lamps and ballasts, probably hundred of them in stock, but my country voltage is way too high at 247V and where the ballast is rated 220V, aside from using a variac regulating transformers, what are the cheaper alternative to drive the voltage down to 220V?

I also find overvoltage does something strange to lamps, my MK4 Philips SL does completely the opposite of what a normal SL lamps would do, it's Poland made and the etch says 230V, the lamp will start at full brightness then gradually dims down after a few hours lol

Try looking for a buck transformer. I'm not sure if they would be much cheaper than a variac though, at least the high amperage ones.

Or a "220/2×xxxV", with the secondaries connected in series with the primary to form an autotransformer. E.g. 2x15V will give you about 7% steps,...
Even a single 24V secondary transformer will bring you to about 225V.
You select transformer with secondary current rsting corresponding to your load current rating need. Count with fact these are designed with turns ratio corresponding to a bit higher secondary voltage, to count with the drop on the internal resistance. In your case the drop will work in the opposie direction, so if that "230/24V" transformer has 26V OCV, connected as the step down autotransformer it will exhibit 247*230/(230+26)=221.9, after loading with the rated current it will drop its 2V so become the required 220V.

Such transformers are regular commodity products, so quite readily available and for relatively cheap.

But there is one pitfall you should take into account:
If the transforme you will be using has a build in safety device (thermal contact, PTC,...), because that is connected to the primary and that in your case is not the complete power input, it is ineffective. More over it will very likely destroy itself or tge transformer once it triggers. That happens, because the "secondary" still remains in the circuit in series with the load, so when breaking the primary (by the built in cutout activation,...), the transformer will attempt to step up the mains voltage with the reverse ratio (so hypothetically create 2.3kV from 240V). Of course the core will saturate (that is not the oroblem), but it will still form those high voltage spikes across the protection device and that is not designed for anything like that.
So the best for your case would be to use a transformer without any build in protection devices and in any case make the required protections (overcurrent, thermal cut out) around yourself in the way these really cut out the incoming power.
In case you only have access to transformers with build in protections, you have to oversze them and set your protection devices (mainly the thermal cutout) in a way the ones build in the transformer won't activate.