Before I start, I'm not so familiar with SPG hotels so feel free to chip in or correct.
Spg has 2 ways to redeem, (
there are other ways but not relevant here).
1. Regular rewards, a certain amount of points gets you a free night.
2. Cash and points, together with a part cash payment a free night is a lot less points. However, this redemption method is capacity controlled.
I'm presuming that since you're going on vacation you don't want to stay in a cheap airport hotel. So here are the rates for Category 4,5 and 6. All prices are converted into US dollars as of today.
RomeWestin (cat. 5) $342 p/n, 12k *points, cash and points not available.
Hotel Eden (cat. 6) $270, 20k, 8k+$150
St. Regis (cat. 6) $342, 20k, n/a
VeniceWestin (cat. 5) $220, 12k, 4.8k+$90
Hotel Danieli (cat. 6) $310, 20k, 8k+$150
Hotel Gritting Palace (cat. 6) $340, 20k, n/a
ParisLe Meridian Montparnesse (cat. 4) $185, 10k, n/a
Le Meridian Etoile (cat. 5) $225, 12k, 4.8k+$90
Westin (cat. 6) $320, 20k, 8k+$150
Prince de Galles (cat. 6) $370, 20k, 8k+$150
So in total e.g. Rome 3 nights @ cat. 5-$1026/36k *points. No cash and points
Venice 3 nights @ cat. 5-$660/36k/14.4k+$270
Paris 3 nights @ cat. 5-$675/36k/14.4k+$270
Grand total: $2341/108k *points/64k+$540
This is basically all the info. You have to go through it and decide for yourself want you want from this trip and how much you are willing to spend, if anything.
Post back and then we can go further.