Complete and partial dentures can be the cheapest but not always the best way to replace missing teeth. The complete denture base is made out of acrylic which holds the porcelain/plastic teeth. The partial denture base can be a cast metal framework with acrylic or a flexible base. The cast metal partial is sturdier since it can be designed to properly distribute chewing forces between soft tissues and teeth. The flexible partial can be a good option occasionally including when aesthetics are important.
The partial and complete dentures can be a great option in the interim when the patient is waiting to receive definitive treatment. e.g. when patient is waiting for surgically placed implant to heal.
When dentures are meant to be used long term (more than 1 year) after extracting teeth a healing period of around 6 weeks is recommended for the extraction sites to heal. The jaw bone shrinks rapidly within first 6 weeks after teeth extraction. The impressions for dentures are made after satisfactory healing. Not everyone can go around without teeth for 8-10 weeks! So immediate dentures can be placed the day of extractions, but a base reline OR fabrication of new denture would be recommended not more then 6 months after immediate denture was delivered.