The flights from Luton are on Mondays only. (The outbound flight leaves at 1410 so you'd have time to get there from Cardiff if you can find any cheap rail fares - or if you don't mind a long drive. The return flight, however, gets back too late, 2320, to catch a train back to Cardiff). Flights for the first half of the month are expensive but the outbound flights on 17/04 and 24/04 are �68.49 and �43.49 respectively. Return flights on 24/04 and 01/05 are �103.49 and �53.49 respectively.
http://www.flymonarch.com/
Your best bet, however, might be to travel via Gatwick. Once again, the first half of the month is expensive but there are outbound flights on Wed 19/04, Mon 24/04 and Wed 26/04 (1455 departure) all for �50 each INCLUDING taxes and charges. There are also some quite cheap flights back. The best of these is on Wed 26/04 (arriving back at 0005 Thursday) for just �50, once again including taxes and charges. So you could travel out on the 19th and back on the 26th for a total of �100 (or more accurately, because of the weird way BA's computer adds up, �99.40). There are, however, several other options available with only slightly dearer flights.
http://www.gbairways.com/
I'm sorry that I've not been able to find any flights nearer to Cardiff. They simply don't seem to exist. I hope, however, that something here will be of help to you,
Chris