For $30-$50 you can't even buy good article from native writer, so don't even think about quality sites for this price where you will publish guest post. There are tons of websites who look like a nice guest posting opportunity but if you check them carefully their latest posts are all paid / sponsored its nothing better than a PBN, i would say that PBN would even give you better and faster effect than such sites. Guest posts are amazing if you post them on targeted and already successful niche websites and for this you have to pay few hundreds or more, or have good connections with bloggers who will publish for free if you provide amazing content to them not just some average article.. So unless you can invest few thousands in campaign every month to hire professionals who will work for you or to buy posts yourself, better stick with PBNs and black hat methods until you make money to push your other projects. With few guest posts you will not make some good results, but if you still thinking to buy something be careful many services are not honest asking 5 times higher prices for something thats not worth, so better check quality of sites (use similarweb for traffic analysis at least 50K to have) if it shows nothing, and you see many paid and random posts, i would say don't post anything on such websites.