Hi Jaana, what sort of hotel are you looking for?
Beaufort House, 65 Cathedral Road, (0)29 2023 7003, is a nice central guest-house where a single room with en suite bathroom and breakfast included is £49.00 per night for that period.
Penrhys Hotel, 127 Cathedral Road, (0)29 2023 0548, is £35.00 per night for single room with en suite bathroom and breakfast.
If you want something more basic, Cardiff Backpackers Hostel, 96-98 Neville Street, (0)29 2034 5577, is £16.50 per night in female dormitory accommodation (6-8 beds) or £25.00 per night for a single room (both with shared bathroom facilities).
These are all within walking distance of the centre of town.
There might be cheaper hotels on Cathedral Road, I’ll have a look in the next few days, here is a page of Cardiff hotels.
The Tuesday salsa scene might be different by the time you get here, Callaghan’s and Zync are both cheap and friendly, but Callaghan’s classes tend to start late and I don’t know what Zync will have on that night.
I would do the castle on Tuesday, it will give you something to do straight away.