Few things:
1) Synthesis in your garage, no matter how legal it is, will probably be perceived by local law enforcement as totally illicit. The only people they know doing chemistry at home are junkies.
2) If a molecule costs $4500 for a small amount, that probably means it's exceptionally difficult to make. You won't likely have much better luck.
3) Plenty of companies exist that sell small scaffolds, but typically they offer a healthy discount to an organization or university, which is a perk you will not enjoy working out of your garage (and these discounts can be hefty depending on your institution). Also, if you're looking at an intermediate that costs $4500 for milligram quantities I'm willing to bet you're looking at an extremely advanced and complex intermediate. That means two things: a) you won't find a "cheap" building block unless you go far back into the synthesis (far before the challenging steps that make the price what it is), and b) if you do decide to make it yourself, you will probably need large quantities of starting material, possibly in the kg range depending on what you're making.