Very rough idea
Slapmarker around £70, buy once with some ink, no more expense
Tagger tool around £20, tags around 50p each
Slapmarker - don't need to handle pig
Ear tag - need to lift ear to apply
Both easiest done whilst pigs are eating
Slapmark can be done early, suggest you don;t tag until just vefore they go, as they can pull then out - pig with damaged ear to look after, 50p wated tag plus have to do it again.