多少

几 usually signals that you expect a small number under ten, like 几个人. 多少 works for any amount, whether the number is big, small, or unknown. Also, 几 needs a counting word, while 多少 often skips it in casual speech. If you are unsure, 多少 is the safe choice for daily life.

几岁 is normal for kids and sounds cute, not insulting. Asking an adult 你几岁 can land as rude or completely absurd in real life. For adults, 你多大 or 您多大年纪 is the polite everyday default. 多少岁 is acceptable, but it sounds more clinical and less friendly.

多少钱 works for any price, from a bottle of water to a new car. 几块钱 only asks 'how many yuan' and hints you expect a small bill. In standard Mandarin, 几 cannot sit alone before 钱; it needs a counting word like 块. If you want the safe phrase for shops, use 多少钱.

“买东西，多少钱？” (When shopping, ask 'How much is it?')

多少 asks quantity, not degree, so it cannot directly modify an adjective. Chinese uses 多 plus an adjective for 'how [adjective]': 多贵, 多大, 多高. If you want a price, stick with the fixed phrase 多少钱.