Your not paying anything for them, your just deducting it from their gross pay and being trusted to hand it over to the revenue every month... but..... when they gross over £110 pw (i think it is) you then have to pay 11% employers NI on anything above this figure, so this is when it costs you a few bob, this would entitle the employee to ssp, smp which you would then offset from your paye payments each month, so all it actually costs you is holiday entitlement which they still accrue at a min of 5.6 weeks pa including bank hols, which is a right old con for you if someone is long term sick and your a small company
I sub out wages to a payroll company as its well worth the small cost. Tax is tax, simple, its worked out on their earnings, you just collect it for free
and pay it to the revenue.