When it comes to electric bikes the price is in the motor and especially the battery. Frames and components benefit greatly from economies of scale. Large manufacturers place huge orders and get great deals on all the pieces. That means if you are converting a bike, you already own, you are purchasing the expensive pieces for the conversion. In turn, that means you might not get much of a deal on an electric bike conversion.
If you are considering converting a bike you own to electric power only for cost savings, think hard about it. In most cases, it's just not going to be worth it. Not only that but a great conversion kit on a not great bike isn't going to be a lot of fun. There are some inexpensive electric bikes out there that are excellent to ride so if the price is the driving factor that's a better option.
That doesn't mean you shouldn't convert a bike you own. If you have a good bike you love then consider a conversion. Finding a new bike can be a challenge and sticking with what works has advantages. Just know, it might not be a cost-saving.