Can you possibly market an rpg cheap, by yourself?

A good tool for free marketing, like it or not, is TikTok. The TikTok algorithm basically ensures that your video will be pushed to at least 10 people. Posting regularly will get your videos in front of more people and you will have an established niche. If you want to get people interested, create some videos of gameplay. The D&D and Tabletop RPG niche is quite large and active, so if you create videos of ridiculous moments in your RPG it will likely resonate with some people there. TikTok has some great tools to interact with viewers and answer questions, so if someone expresses interest in your game you can inform them of where to get a hold of the guide.