Good tripods typically come without a camera head, with 3/8" head attachment.
Then you purchase a separate 1/4" camera head for it. The reason is that
different tripod owners may prefer different types of heads, and it doesn't
make sense to fix a head to a tripod so that they can't be changed.
Interchangeable heads are better. If a tripod doesn't have an interchangeable
head, it's a sign that it may not be as good as possible.
The lens, 75-300, is quite long. On a Canon 40D, it's 480mm equivalent at the
long end. Not only is it heavy, but also it suffers from vibrations more than a
wide angle lens due to magnifying the image more than a wide angle lens.
I have a Velbon M43, which is suitable in my opinion up to 50mm lens on a crop
camera if used with 2 second self timer or remote release, but even
photographing the moon with a lightweight 55-250mm lens at 250mm was too much
for the tripod. The vibrations were huge when I magnified the image by 10x for
manual focusing! A 300mm lens would be even more horrible with the M43. You get
what you pay for.
The answer is that 100 USD will buy you a used quality tripod without head and without a carrying bag. I paid 70 EUR (79 USD) for a used
very old Manfrotto 190 tripod without head (a new one would cost 220 EUR / 247 USD but then again the new model is better than my very old model). The 190 is much better than Velbon
M43 at 250mm, I have already tested it and am very satisfied. It dampens the very minor vibrations very quickly. When the next
full moon comes, I am ready for photographing it.
I paid 62 EUR (70 USD) for a new Manfrotto 496RC2 head, and am planning to
purchase a 50 EUR (56 USD) carrying bag for the tripod. That makes the total
price 182 EUR (205 USD).
Note that quality has a weight: you can't expect your tripod with head to be
much lighter than 2 kilograms, because at this price point carbon fiber is not
an option. For a long lens, it isn't overkill to look for a tripod with load
rating of 4 times the weight of your equipment. Canon 40D with 70-300 is about
1.5 kg, so I would say 6 kg load rating would be good for little vibration
at the long end. (My very old Manfrotto 190 has 5 kg rating, my camera with
55-250mm is 0.85kg, so there's nearly 6x margin in my use case.)
So, the answer is that 100 USD will buy you a used quality tripod, but not with
all the other stuff you need. No way will 100 USD buy you a new tripod with
head and carrying bag suitable for 300mm lens on a crop camera. The new tripods
costing less than 100 USD are NOT suitable for a 300mm lens on a 1.6x crop
If you're planning to use the 300mm end of the lens often, I would consider
going a step up from Manfrotto 190 and look for a used Manfrotto 055, because 300mm is longer than the 250mm with which I tested the Manfrotto 190.
Naturally, there are other brands worth considering as well.
If you're very lucky, you can find a used quality tripod with head for 100 USD.
If you're even luckier, the head is of the type you want. Carrying bag is something optional, so if you're tight on budget, you can skip the carrying bag.