Yasen class is a nuclear sub (SSN). It costs the equivalent of US$1.6 billion. It is a 8600 ton sub. It is one of the largest and most sophisticated attack subs the Russians make. India was trying to 'lease' a few examples in case their Arihant SSN sub development was delayed. It is out of the question for Bangladesh to purchase such a sub and will probably not get it due to strategic reasons.
The Chinese also make SSNs and SSBNs such as the very large Shang and Jin classes which carry ballistic missiles but China does not 'export' those subs because of strategic reasons. Bangladesh' next purchase will probably be the export version of a Type 039A sub which is a conventional Diesel-electric engine sub. It is about the size of the Kilo class, slightly larger.
The Thais and Pakistanis are buying this AFAIK and I believe this are the versions Bangladesh will probably get. These will cost Thailand and Pakistan $330 million or so each independent of spares and support. In April 2015, Pakistan approved purchase of 8 S20 class from China for $5 billion. The Thai designation is S26T, they are buying 20 subs. These are conventional Diesel-electric drive with 1 shaft/propeller, and AIP. They are ideal for Bay of Bengal usage and mission for BN.