How much it costs you depends on how much you are paid, but it will be between 5.5% and 12.0% of your pay.
Your rate will depend on the level of your pensionable pay, even if you work part time
The real cost to you will be less because you do not pay tax on the amount you pay into your pension. You may also pay a lower rate of National Insurance.
If you work part time or term time you will only pay contributions on your actual pay.
Alex works full time and his pay is £20,000 a year so he will pay 5.5% of £20,000.
This works out at £92 a month, but the real cost to him will be nearer £72 after accounting for tax savings.
If he worked part time (half the hours) he would still pay 5.5%, but only on his £10,000 pay