One of the first steps towards the effective Technical Debt (TD) management is the quantification and continuous monitoring of the TD principal. In the current state-ofresearch and practice the most common ways to assess TD principal are the use of: (a) structural proxies—i.e., most commonly through quality metrics; and (b) monetized proxies—i.e., most commonly through the use of the SQALE (Software Quality Assessment based on Lifecycle Expectations) method. Although both approaches have merit, they seem to rely on different viewpoints of TD and their levels of agreement have not been evaluated so far. Therefore, in this paper, we empirically explore this relation by analyzing data obtained from 20 open source software projects and build a regression model that establishes a relationship between them. The results of the study suggest that a model of seven structural metrics, quantifying different aspects of quality (i.e., coupling, cohesion, complexity, size, and inheritance) can accurately estimate TD principal as appraised by SonarQube. The results of this case study are useful to both academia and industry. In particular, academia can gain knowledge on: (a) the reliability and agreement of TD principal assessment methods and (b) the structural characteristics of software that contribute to the accumulation of TD, whereas practitioners are provided with an alternative evaluation model with reduced number of parameters that can accurately assess TD, through traditional software quality metrics and tools.