I have a Tamron Adaptall SP LD IF 300mm f2.8 that I bought via ebay and they come up every now and again and are a great buy with very optics and are a great astroimaging lens . I has a AD2all - EF adapter made for it so I now have that and the AD2all - MD adapter so I can use on my 40D or my venerable XD5.
I picked up a Canon 300 f2.8 hardcase to put it in too.
All under $800 which beats the price asked for secondhand Canon and Sigma 300mm f2.8 lenses.
If you see one advertised on Ebay, bid on it, as these are first rate.
A nice cheapy is a Soligor 135mm f2, I've one with a FD mount that I use with FD-EF adapter what works very nicely for astroimaging on the 40D (Becomes 200mm f3 with the adapter). There's was one on Ebay the other day
http://cgi.ebay.com.au/ws/eBayISAPI....=STRK:MEWAX:IT, .
Another good lens to look out of is a Tamron SP
LD IF 80-200mm f2.8 adaptall-2 30A. Scarce as hens teeth and another top astroimaging performer from what I've read.
Look them up on Ebay and see what they sell for, that would give you a good indication on how much they are worth as secondhand lenses.
The Tamron 24mm f2.5 will have a max apeture under 10mm , so not so useful for astronomical imaging IMO.