Illumina sequencing requires costly enzymes and technology in order to sequence via PCR chemistry.
There is a newer NGS platform that is extremely cheap in comparison. Nanopore sequencing by Oxford Nanopore Technologies is a great option, especially for 16S sequencing. Look into the MinION for a cheap option. They even offer quick analysis tools post-sequencing to help with data analysis of metagenomic samples.
All Next-Gen sequencing has been dropping in price over the last 10 years. Dramatically so in the last 5. Any choice of NextGen sequencing is much cheaper than cloning and Sanger. There are many competitive options (Illumina, Life Technologies, Roche (454), PacBio, and Oxford Nanopore, etc.)
It may be locally expensive for you if your sequencing folks don't do a lot of runs on the machine. Shop around and see if you can get a better price from another facility. Make sure to ask when preparation you need to do in advance as the sample prep can get expensive and how they would send you the data.
Note that some other manufacturer's provide alternative to illumina library preparation kits (New England Biolabs, BioScientific, and other...). These kits provide varying performances but if you add to your process some adapted controls, I think that these kits could help provide sufficient decrease of your sequencing costs.