There are some good bargains to be had on the used market, though you may have to do a fair bit of searching! Not particularly light, but a friend of mine picked up a Gitzo G1327 at the local camera club for 100 pounds and it was almost mint - I hate him!
He also has one of these Feisol tripods which is pretty good and VERY light:
http://www.feisoleurope.com/feisol-tournament-tripod-ct3342-rapid-p-30.html
A little over budget but not too much.
This is just a personal view but I find spotting scopes (I use a Kowa TSN4) to be more demanding of tripods than camera lenses. I use a Canon 800mm F5.6 L IS lens and it is much easier to support than my scope. Its weight damps down vibration and the magnification is much lower. With a spotting scope you don't have the weight or any, built in, image stabiliser.
Light weight tripods are nice to carry but can be disappointing when it comes to viewing.
Just a thought, did you know that you can use a number of the higher quality Opticron eyepieces on your scope. I am currently using the Opticron SDL V2 on my TSN4, and it works quite nicely! You will need to add a spacer (cheap neoprene O ring) but they work well.
http://www.opticron.co.uk/PDF/SDLv2_Eyepiece_User_Guide.pdf