On their taxes? Conor claims he spent 300,000 on training. Not sure how he would do it seeing he is from Ireland. But how does it work for American fighters? 
I know in Australia you can claim costs that help educate you for your job (eg: a food hygiene course if you work in a kitchen).
Maybe training to learn to punch and not get punched is claimable in fighting? 
As a Self employed person I would put anything even remotely connected to work as an expense and pay for it before tax, I'd imagine fighters do the same.
I'd also guesss that fighters like Conor, with there own promotion companies, Nick, Chael etc. would use the promotion company to pre-tax every expense,
E.g Conor rolling around in a Rolls is part of Mcgregor promotions
Chael flying to LA is part of Submission underground promotion
Where a nexus between the incurrence of an expense and the derivation of income exists, the expense is usually deemed to be deductible unless specifically excluded. This principle is generally applicable across the majority of jurisdictions (around the world).
In Australia, an individual deemed as being a sportman/woman is permitted to average their professional income (derived from sport) under subdivision 405 ITAA 1997 (i.e. This is a concession that is meant to decrease a person's income in the current year where such income exceeds their derived income in prior years, as the income stream is deemed to be abnormal).
Further the deduction would be deemed allowable under section 8-1 ITAA 1997 as it would satisfy the positive limbs of that section.
'always consult with a tax expert'
Generally yes. Grey areas where an accountant can help would include food and supplements, but trying to claim a deduction for winstrol is prob not a good idea.
Trainers and coaches also would be deductible.
I imagine a fighter who owns their gym would be able to do some really creative stuff as well.
ziad - Where a nexus between the incurrence of an expense and the derivation of income exists, the expense is usually deemed to be deductible unless specifically excluded. This principle is generally applicable across the majority of jurisdictions (around the world).Insert blank stare meme.
In Australia, an individual deemed as being a sportman/woman is permitted to average their professional income (derived from sport) under subdivision 405 ITAA 1997 (i.e. This is a concession that is meant to decrease a person's income in the current year where such income exceeds their derived income in prior years, as the income stream is deemed to be abnormal).
Further the deduction would be deemed allowable under section 8-1 ITAA 1997 as it would satisfy the positive limbs of that section.
Trainers and equipment yes
Yes