Here's the analysis based on my daughter's current 1 x 10speed setup:
Current 1 x 10 setup
| Component | Type | Weight (g) | Cost |
|---|---|---|---|
| Cluster | XT 11-36t | 343 | $50 |
| Shifter | XT 10 speed | 118 | $50 |
| Rear derailleur | Zee 10 speed | 270 | $60 |
| Totals | 731 | $160 | |
XT 1 x 11 setup
| Component | Type | Weight (g) | Cost |
|---|---|---|---|
| Cluster | XT 11-40t | 411 | $110 |
| Shifter | XT 11 speed | 120 | $65 |
| Rear derailleur | XT 11 speed | 273 | $110 |
| Totals | 804 | $285 | |
XT/XTR 1 x 11 setup
| Component | Type | Weight (g) | Cost |
|---|---|---|---|
| Cluster | XTR 11-40t | 329 | $210 |
| Shifter | XT 11 speed | 120 | $65 |
| Rear derailleur | XT 11 speed | 273 | $110 |
| Totals | 722 | $385 | |
Bottom line, for about AUD$285 you can convert to 1 x 11, get an extra gear and add about 70g in weight or spend $385 and come in at about the same weight as the 1 x 10 XT setup. There are certainly no weight savings in going 1 x 11 if you have sufficient gear ratios in your current 1 x 10 setup.
What about other options?
If you really need the extended range, then adding a 40 or 42t cog to your existing 1 x 10 setup may be the go.