For a range look at SOLD listings on ebay.
HP-100s are either 100W or 200W, fthe 200W have silver rings around the bass port and are slightly more desirable, worth $300 for a decent working pair, more if excellent.
The turntable I believe is around $150, depends on condition also and cartridge and needle quality and condition, a very desirable cartridge can add significantly.
The cassette deck I don't know anything.
The SX-1250 is very condition dependent. Perfect cosmetics and fully restored over $2,000. A normal working unrestored with no significant cosmetic problems and nothing more than maybe a crackle in the volume pot is still $1,000+
Best to look at ebay sold listings, realize that this is adding the international market of ebay and local sales are significantly less, and be brutally honest when comparing condition / restoration / history.
Oh, and if you have manuals and boxes, big bonus in value, people love buying original owner gear with boxes and manuals.