I have recently helped a few foreigners in a similar situation (wanting the lowest cost funeral). Where the actual death happened at the hospital where the person was rushed to at the end stage, a death certificate was easily issued, and on the basis of this, a police clearance was issued. Where the foreigner had died at home, the police came for a routine enquiry and spoke to the neighbours and concluded that there were no suspicious circumtances and the autopsy was waived and police clearance issued.
At the administrative office of local temple, we informed them of our requirement for low cost funeral and submitted the originals + copy of police clearance. For a sum of 11,000 Baht, a few monks chanted prayers for 10 minutes and the body was cremated thereafter. An official cremation certificate was issued.
Next day we collected the ashes/bones in a ziplock bag and immersed it in the river. Last bit was going to the embassy with police clearance + cremation certificate to get the passport cancelled. Pension was automatically stopped on passport cancellation. This was at a temple somewhere in Onnut Road in Bangkok.