OD&D (v3, pg.22) and AD&D (pg. 29) both include tables outlining costs of expert hirelings. You might decide that hiring an engineer-architect and a blacksmith might be required to build the forge, etc., then add laborers and extra costs around gathering and preparing the lumber etc. My assumption being that hiring the specialist would amount to hiring a foreman who would handle the details and present the player with the bill.
If there are no experts, perhaps back up with some questions about how well the PCs would be able to design, plan, and execute - for instance - a forge. There are dependencies for a lot of technologies you might have to take into account. Would your character even know how to build a proper wall? Know not to build with green wood? How to build something that can generate and withstand the heat needed to melt iron? If there is no civilization to around are you already supplied with adequate tools including shovels, axes, grinding stone, picks?
A lot of things end up requiring surrounding infrastructure. Example: you want to make a 200-page spellbook. Forget magic inks and rituals: first you need the skins of 14-15 animals (goats, sheep, or calves). And even that assumes you have an experienced tanner who knows how to prepare the skin to make leather and parchment. Even if you can find a local who will sell you that many of their flock (which they rely upon for income/food/warmth, etc.) it will still take months to prepare the skins.
In short, depending on the goal, I would make it very difficult for players to build or reconstruct a civilization on their own - UNLESS (of course) it is more fun or expedient for the game to make it more simple.
Happy to kick more ideas around if you have something specific in mind.
