HI All
I live in a condo . Our hottub heater conections got loose and burned up the insides of the control box. The heater is 15 plus years old so we bought a new unit. We replaced the wires from the gfci too the control box on top of the heater with #6wire

My question is if I'm drawing 36+ amps do I keep the 40A breaker or buy a new 50A one for $250+
The heater is 240V 10Kw that would draw 42amps
BUT we have 208V supply only which works out to 36.1111A verified with a meter 36 and 38 per leg

A 50 amp breaker would be the way to go... but where did you get that $250 figure?
Usually the mfg of the tub will dictate what size circuit to put it on.

By the way, the entire circuit needs to be #6 wire, not just from the 50 AMP GFI, assuming the GFI is not at the panel.

And, if this condo is part of a multi-family dwelling, I doubt that your local/state will allow an unlicensed person to do electrical work in this building.

It should be a GFCI breaker and some brands could be 250.00. Definitely 50a if it were me, at least #8 copper/#6al and a #8 cu/#6 Al ground in any case. Immersing yourself in water with electrical involved is definitely a job for a qualified electrician.