Think about weight and bikes a bit more and you'll realise that it's the total weight of you plus your bike that needs to get up hills.
Let's assume a 150 lbs rider and a 25 lbs bike ... the total is: 175 lbs.
Now assume a 150 lbs rider and a 15 lbs bike ... the total is: 165 lbs.
The difference between a 25 lbs bike and a 15 lbs bike is probably $3000 ... the difference in total weight is only 6%.

Sure ... a lighter bike will help you get up hills faster ... but simply training a bit more and improving your technique will have a much bigger improvement than shelling out $1000.