According to glassdoor.com, the average PM salary is $66,048 while the average programmer salary is $43,658.
However, the relative salaries are going to depend greatly on perceived programmer skill, PM duties, company culture and negotiation skill.
Successful project management is a science and in demand skill in its own right (PMs require both project management skills, something which there are a number of professional certifications for, like PRINCE2 and PMP, and a bit of knowledge about the domain the project is in), and it sounds like your company may not fully appreciate what a good project manager can do.
However, project management is a difficult field to get into; you need experience to get a job, and you need a job to get experience, so if you want to be a project manager elsewhere, an offer to get a first job in project management is valuable even if it means temporarily getting paid less. I'd suggest reading some literature (e.g. the PMI's Project Management Body of Knowledge, Managing Successful Projects with PRINCE2, The Agile Manifesto, and books on popular agile methodologies like XP, Scrum and so on). If it sounds like you, taking the job could be a good opportunity, because it will help you to get a PM job elsewhere. Don't forget that developing yourself as a good PM takes just as much effort as a programmer, so keep up with opportunities to learn new methods and get certifications.