Doing another undergraduate degree or even a two-year law degree could be exorbitantly expensive, especially for an international student; the PGDL, however, is a very financially viable option - all courses cost under the £15,000 mark. Many people who have qualified abroad and hope to work in the UK opt for the PGDL.
Don’t get me wrong, you are right, financially speaking, self funding the PGDL over a second undergrad or a two-year LLB is extremely viable.
Also, it is a good option depending on your career plan. After the PGDL you can find a paralegal role to get work experience, network and fund your time in the UK until passing a TC, as I mentioned in the end of my answer.
However, even for those graduating from commonwealth countries or with large work experience overseas, it is hard to compete with local undergrads to get TC, NQ or associates roles. The PGDL (or even a LLM with a distinction) will not make such a difference in your TC application as the firms will look primarily at your undergrad marks. It can show your commitment to law, but don’t compensate undergrad marks.
So saving money now can cost your time in the UK if you don’t get a TC or a paralegal job right after the PGDL.