If the valves are damaged it'll be because they've hit the tops of the
pistons and been bent as a result.... meaning the pistons also likely
to be damaged; so too are the vale guides, in fact this kind of event
can be fairly catastrophic. You'd know if they'd hit because the noise
would have been dreadful and the engine would now have very poor
compression and run poorly. I'd guess it would take a decent mechanic
at least a full day's worth of labour to sort, maybe even a couple of
days, assuming he simply does a complete cylinder head exchange,
replaces pistons etc... but along the way you'd have the costs of the
new parts plus re-grinding the cylinders etc. You're probably looking
at some serious costs here, maybe more than the vehicle is worth and it
may be cheaper to get a recon engine. Before doing anything check the
compression (or get it checked). That'll give you a fair idea of any
damage.