Perhaps you might want to try this 24hr guarded parking:
http://www.parkovisteladvi.cz/images...vi_final+I.jpg
which is only 100m from the Ladvi (C-line) metro station. Their rate is 30 CZK (5:00 - 20:00) and 80 CZK (20:00 - 5:00) so in total you will pay 110 CZK (4.25 EUR) for every 24 hours.
http://www.parkovisteladvi.cz
GPS:
50°7'32.893"N, 14°28'1.448"E