Oricât de is a common Romanian pattern meaning however or no matter how before an adjective or adverb.
So:
It introduces a concessive idea: the main action still happens regardless of the size of the fee.
Because taxa means the fee, while taxă means a fee.
Romanian usually puts the definite article at the end of the noun:
So taxa de participare means the participation fee.
De participare specifies what kind of fee it is.
Romanian very often uses de + noun to make this kind of noun phrase.
Este is the 3rd person singular form of a fi (to be), and it matches taxa, which is singular.
So:
In everyday speech, many Romanians would also say e instead of este:
Both are correct; este is just a bit fuller or more formal.
The first part, Oricât de mare este taxa de participare, is a subordinate clause. The second part, o plătim, is the main clause.
Romanian normally separates this kind of introductory concessive clause with a comma.
So the comma marks the division between:
O is a direct object pronoun meaning it here.
It refers back to taxa, which is feminine singular, so Romanian uses o:
So o plătim literally means we pay it.
In Romanian, short direct object pronouns usually come before a finite verb:
So o plătim is the normal word order.
Romanian often leaves subject pronouns out because the verb ending already shows the person.
Here, plătim means we pay:
So noi (we) is not necessary, though you could add it for emphasis:
Not in this sentence.
With a normal finite verb in an affirmative clause, Romanian uses the clitic pronoun before the verb:
You may see pronouns attached after the verb in other situations, especially with imperatives:
But here, o plătim is the standard form.
Yes. That version is also natural.
There is a slight nuance:
Both are good Romanian. The version with ar fi sounds a bit more abstract or concessive.
Literally, mare often means big, but with prices, costs, fees, and amounts, English usually translates it as high.
So:
This is a normal meaning extension in Romanian.