hydrogen vs helium for weathue baloons.
I posit h should used rather than he
my resoning is
1 he is rare and finite and expensive.
2 we can make all the h we want
3 h is lighter and has more lift so smaller baloons can be uaed.

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He: [1] costs: 6...7 €/m³ = $6.51...$7.6 / m³ = 18.43...21.52 ct/cuft (wholesale price)

large weather balloon: weight: 3,000 g (6.61 lb); payload: 3,000 g (6.61 lb), volume: 6.3 m³ (222.48 cuft) @norm pressure
---> diameter: @norm pressure: 2.3 m (7.55 ft); burst diameter: ~13 m (~42.65 ft) in stratosphere @~40 km (~131,234 ft)

He for balloon costs: 18.43...21.52 ct/cuft * 222.48 cuft = $41.00...$47.88 for each balloon

H (cheapest "gray" hydrogen through steam methane reforming):

3 €/kg = $3.255 / kg = $1.45 / lb. 1 kg H @norm pressure = 11.24m³ --> $0.2896 / m³ = 0.82 ct/cuft

--> He costs about twenty-five times more than H
--> lower density of H... higher payload....

Source:
[1] (German) Federal Institute for Geosciences and Natural Resources (BGR)

...BUT:

Flammability of Hydrogen is quite an issue, not just in the ground handling, but also in the low altitude segment of the flight.
The cost of inflating a "professional" balloon is not really a factor in the cost of flying a balloon - the payload must be considered expendable and will vary between a few hundred dollars (single instrument pack, with minimal data transmission or storage) to several thousand, depending on the nature of the science pack and the intended flight duration.

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Bob Smith
Member of Seti PIPPS (Pluto is a Planet Protest Society)
Somewhere in the (un)known Universe?

found a calculation from a school project to start a weather balloon:

• balloon, 1.6 kg payload; parachute, GPS tracker, styrofoam box: 500 €
  
• science pack, data logger: 300 €
  
• insurance: 100 €
  
• ascent permit / aviation authority: 100 €
  
• Helium gas: 200 €