Aquest means "this" and aquell means "that", and both agree with the noun: aquest/aquesta/aquests/aquestes. To ask a price, use quant costa? for one thing and quant costen? for several.
Examples
- Quant costa aquesta camisa?
- How much is this shirt?
- Vull aquell, no aquest.
- I want that one, not this one.
- Quant costen aquestes sabates?
- How much are these shoes?
The mistake to avoid
Costa or costen follows the thing being bought, not you.
Where this fits in
Food & ordering, Catalan A1: Order at a café or restaurant, ask for the bill, and talk about basic food and drink.