What is a reasonable price to spend on a bayonet mount ring for my tripod?

Once mounted on the tripod you can loosen the ring to swivel the camera (and lens) from landscape to portrait position.

A bayonet mount allows lenses to be changed quickly and ensures they are locked into place

Well, I have never needed to purchase my bayonet rings in bulk before; however there is a great website that should have just what you are looking for. I will post the link for you here - hope it helps.

http://www.alibaba.com/showroom/bayonet-adapter-ring.html

I like to buy things for my camera from eBay. They usually have a selection of what I need at fair prices. I have found a bayonet for a Nikon camera and will post the link here for you too.

http://www.ebay.com/itm/NIKON-18-55-18-105-18-135-55-200-BAYONET-OEM-/140531628517

There is a great video on YouTube that will provide you with a thorough tutorial that will guide you through replacing your bayonet. I will post the link for you here.

http://www.youtube.com/watch?v=Pd6pvvQ1_So

The mount has a 1/4-20 screw that fits most camera equipment. If you have the scope (spotting scope) it should have that hole in the bottom of the mounting ring. If the screw is missing from the window mount then most 1/4-20 tripod screws will work or you may substitute a cap screw and flat washer, to work with an Allen wrench for tightening.

Do NOT buy that telescope.

Second, to take long exposure (several minute) astro-photos you must use a GEM mount like one of these:

http://www.telescope.com/control/accessories/mounts-and-tripods/orion-sirius-eq-g-goto-telescope-mount

http://www.telescope.com/control/accessories/mounts-and-tripods/orion-atlas-eq-g-computerized-goto-telescope-mount

Then you need to decide on the scope to use. Something like this would work:

http://www.telescope.com/control/product/~product_id=07338

or this:

http://www.astronomics.com/main/product.asp/catalog_name/astronomics/category_name/U5QNWB3RKWWL8N3EL99F9DX9A0/product_id/AT6RC

Additionally you will need some accessories to mount the camera onto the back of the telescope, and you will need to learn how to polar align the mount so it will accurately track the sky.

Read these articles:
http://www.astropix.com/HTML/I_ASTROP/TOC_AP.HTM

If it's the VR lens:
http://uscamera.com/jaa79851-b37.htm

If it's not the VR lens:
http://uscamera.com/jaa79351-f.htm

As to whether you can replace it yourself, can you use a screwdriver and a soldering iron?

Probably screws that are on the part of the lens mount that touches the camera bayonet

Although I'm not familiar with your particular make and model it sounds like a common problem which affects many other tripods. If the legs clamp with a lever action then if you closely examine each clamp as you activate the lever I expect that you'll find a small crack opens up in the clamp assembly.

It's a faulty design as the clamps simply aren't strong enough for the load and eventually crack due to fatigue. Some manufacturers supply replacement leg clamps but they also fail and fitting them usually costs more than the tripod is worth.

Next time, consider buying a tripod which has rotating collars to lock the legs with. They're not as quick to operate as lever action (a.k.a. "quick release") clamps, but they don't take much longer and last far longer. Rotating collar clamps are only suitable for tubular legs, and to be as rigid as extruded profile legs there's usually an increased weight penalty unless you spend far more on carbon fibre versions.

Personally, I tend to buy cheap but perfectly good non brand name lightweight tripods which I can get for £10 including a carry bag and handle and expect to have to replace them when broken: usually about once a year. It's a lot cheaper than spending over £300 on a good one and means I can afford to have a spare one in the car.

ALDI (in the UK and Europe) sell a £10 tripod which comes with a handle, carry bag, pan/tilt head,quick release leg clamps, geared centre column plus two quick release mounting plates and has a bag hook at the bottom of the centre column; it even has a no quibble three year guarantee. It's not a smooth or as well built as many, but can't be beaten at the price.