(Not a Teacher)
If I were to use reckon in this sentence, it would be like so:
"Gold shares, though far dearer than they recently were, are still attractive to some who reckon the prices will (continue to?) rise."
I don't think I've ever seen 'reckon' used with 'on', but I guess it could work, though.
'Capitalize on' also makes sense, though it still feels like something else should be added to the sentence:
"Gold shares, though far dearer than they recently were, are still attractive to those who seek to (or aim to) capitalize on rising prices."
- For Teachers