Well, there are approaches you can choose from when upgrading.
I tend to try and put a budget on upgrading cheapies and certainly going past the value of the instrument probably busts that; otherwise it might just be a better option to get a better one in the first place.
Since Spirits are sub-£200 instruments (s/h), I would most likely baulk at a pair of £80 pickups.
(Of course, that's my approach - no reason why yours has to be the same!)