| Godwyn |
An oft wondered question. It is pretty much firmly in the hands of the DM as to whether it is allowed at all, and then the pricing. It has to be decided which abilities count as +modifications for pricing, and what abilities should have a simple gold cost associated with them
Part of the problem for the sun blade is determining the exact value and type of some of the enhancements. That it acts at +2 is easily counted, but the +2 versus all evil creatures is a very inclusive bane property, but lacks the 2d6 bonus damage. So possibly a +1 like the bane property.
Double damage and an increased crit modifier is quite potent, even on the subset of foes because the foe list. Maybe gold cost, maybe +1.
For the odd damage die increase for a shortsword (odd because 1d6->1d8->2d6 not 1d10) or using a bastard sword as a light weapon, I can see a +1 also. (Problem with either of these being that I know plenty who would find amazing uses for either ability alone if it was a gold cost).
The daylight is pretty much a daylight spell 1/day, except it actually acts as daylight (clearly to deal with vampires going with the theme of the sword). So 3*5*1800/5 + random for actually being daylight, but lasts less time and requires concentration (effectively), I would leave as is at 5400.
This nets a +4 weapon (32000) +5400 +a cost for the crit modifier. At a +1 bonus the puts the weapon at 55400, just a bit over its listed price of 50,335.
I, as a DM, would just set it as a +5 weapon with the daylight added on for pricing purposes. This would make additional weapon enhancements expensive to place on it, as I feel they should be if allowed.