It depends a lot on the purchase item: it is different fo food and insurance policy, for example. However, I would say it's the perception of "value for money" of an average consumer.
One answer to research further for your studies is the concept of "consumer surplus" - that's the value that consumers feel they receive in excess of the price they pay... a consumer may be more inclined to pay a higher price or choose to buy an item at all if they feel they are getting "more than they bargained for".
I can add to Jayne's answer that you may have to look at the idea of "Willingness to Pay". There is quite a bit about the factors that impact buyer behaviour (such as "theory of behaviour").
Another model that can bring you closer to an answer is the "Kano model". [ Kano, N., Takahshi, F & Tsuji, S. (1984). Attractive quality and must-be quality. The Journal of the Japanese Society for Quality Control ]. The basic premise is that we normally pay more for something that perform better. But some things are considered as must-haves...like an ATM...it must have money. But check it out. I often refer to it.
That depends on what you are looking for, there is a Decision Making Process (DMP). Most business schools have articles related to the process, but pick a model closest to the type of buyer you are engaging. Additionally, there is a model called a "value pyramid", that influences customer behavior. The two are noteworthy in marketing theory.
Aaker et al (2010) have investigated the stereotype content model (e.g., Fiske et al., 2002) on willingness to buy. They found out that non-profit firms are perceived as warm but incompetent, whereas for-profit firms are perceived as cold but competent. The competence dimension predicted the willingness to buy.
You find the Aaker-Paper here: https://academic.oup.com/jcr/article/37/2/224/1814192