Numbers 20-100+ & Prices
The tens είκοσι, τριάντα, σαράντα, πενήντα, εξήντα, εβδομήντα, ογδόντα, ενενήντα are all INDECLINABLE; you compose the numbers between by writing the ten and the unit as two words (είκοσι πέντε = 25). 100 is εκατό on its own; it becomes εκατόν ONLY before a following numeral when forming 101–199 (out of scope here) - a spelling rule to recognise, not a separate number to drill. Prices are simply “number + ευρώ”, and ευρώ never changes: Πόσο κάνει; - Δέκα ευρώ.
The rule
The tens - είκοσι, τριάντα, σαράντα, πενήντα, εξήντα, εβδομήντα, ογδόντα, ενενήντα - are all indeclinable, and you compose everything between them by writing the ten and the unit as two separate words: είκοσι πέντε, τριάντα εφτά. Nothing agrees and nothing changes. 100 is εκατό standing on its own; the longer form εκατόν appears only in front of a following numeral when building 101-199, so it is a spelling to recognise rather than a second number to learn. Prices are the simplest pattern in the language - number plus ευρώ, and ευρώ is invariable, so it is δέκα ευρώ and είκοσι ευρώ alike. Ask with Πόσο κάνει;, where the neuter πόσο is the adverbial “how much”, not the declined adjective πόσος you put in front of a noun.