The info posted here on HGH is a bit misleading. If you do a quick Google search for "acromegaly," one can easily see from the photos that come up that an excess of HGH does affect bony tissue quite significantly. In young individuals (that have not achieved adult height) HGH will lead to gigantism (increased linear growth). After linear growth ceases, HGH causes all tissues to grow, including bone, leading to coarsening of facial features due to the thickening of bones, and enlargment of internal organs. It is no coincidence that people with gigantism or acromegaly can die from complications like hypertrophic cardiomyopathy (enlargement of the heart), liver failure, and other organ diseases. While treatment of HGH deficiency in childhood is well established, and even now some doctors treat HGH "pseudodeficiency" in short children, HGH replacement, even in cases of true deficiency, is not performed in adults. It produces only mild increases in muscle mass at normal doses, and high doses have too great a potential to cause harm.
Hormone replacement therapy is targeted at correcting underlying hormonal deficiencies. Since testosterone falls off with age, it is actually quite common to perform hormone replacement with testoserone gels, patches, or shots. This replacement tends to be at the level of roughly 100mg/week, with the goal of producing a physiologic level of testosterone in the body. This is associated with an increase in lean body mass, decrease in fat, correction of high blood pressure, correction of dyslipidemia, an increase in insulin sensativity, and possibly an increase in vascular endothelial reactivity. It is actually thought that low testosterone is one factor that predisposes aging men to weight gain, bad cholesterols, high blood pressure, diabetes, and heart attacks. Frankly, I think any guy past his mid-40's should get his levels checked and consider doing hormone replacement.
HGH levels do not fall off with age, and thus any therapy with them after linear growth has ceased seems to be of dubious benefit, at best. Any hormone, in excess of physiologic doses, has the potential to cause harm. Again, with testosterone, at the doses taken in a "cycle," which can be up to 10g a week, you end up with a high risk of all the side effects above. This is especially likely for a 20 y/o male, who is naturally around his peak level of natural testosterone production. At this age, your body is already making so much testosterone that all your androgen receptors are already fully saturated. Your body has no need for the extra hormone.
Other things to keep in mind is that the impotence, shrunken testicles, and infertility caused by high doses of testosterone are not necessarily reversible. Depending on how thoroughly you have suppressed your testicular cells, they may not recover when you come off the steroids. I would imagine that it somewhat defeats the point of getting buff if you look hot and can't get it up.
I agree with all the previous posters who said you just need to eat more. It is absolute fact that you cannot gain weight unless you eat more calories than you burn. If you are doing cardio, stop, or at least cut back to just 10-15 mins before you lift. Considering the amount of money you are talking about spending, why not schedule an appointment with a nutritionist and work out a proper diet to gain weight. It would probably be cheaper, and I don't doubt that you would see results.