Yangtze cruises leave from Chongqing, not Chengdu. Questions:
1) Why do you want to do this cruise? Not a trick question, BTW. What's the motivation and your expectations? Many are disappointed especially these days since the Three Gorges Dam has been built.
2) What time of year is this trip? There are definitely times of year when the Yangtze is not at its best...or even visible when fog is thick.
3) What's the rest of the trip look like? What are your other options on the wish list? Most people have a longer wish list than they have time for, and the Yangtze Cruise almost always ends up displacing something better.
To answer your question generally, 3 types of transport on the Yangtze:
a) Foreign tourist-oriented "luxury" cruise boat, like Victoria and some other companies run. The boat is your hotel and your transport. Expensive. You might check out
www.cruisecritic.com for comments on the various companies running these.
b) Chinese tourist-oriented budget cruise boat. These can be pretty basic and pretty dirty but much cheaper. This is the type of boat represented by those lower end quotes you are getting. Again, the boat is your hotel and your transport. There will not likely be any English speaking capability. This is best booked from Chongqing or elsewhere in China from a local agent. Just be clear about what you are getting into. If you are fussy, picky, used to luxury, and with little experience traveling local in developing countries, then forget it.
c) Hydrofoil. These are for day transportation only, but people who live in the towns along the river use them extensively to get up and down. They are fast and have an enclosed cabin with seats, assigned on the ticket. Tickets normally purchased on day of or day before travel. There is a platform with an small open area for fresh air and photo taking, on each side of the hydrofoil. It is possible to see the most scenic parts of the Yangtze--from Wushan town to the 3 Gorges Dam--in only a few hours. Also possible to do an overnight on shore and a side excursion by taking hydrofoil part way one day and rest of way to Yichang the next. There is some ancillary bus transport involved. This is cheapest and fastest way to see the Yangtze but few western tourists know about it. You cannot be heavily laden with tons of luggage but a rollaboard and daypack is OK.
All boats are scheduled to go both upriver and downriver, but the downriver is a little faster. Not all of the big cruise boats go through the Three Gorges locks, most of them offload at a pier upriver and then have road transport arranged to Yichang. Or vice-versa if heading upriver to Chongqing. Hydrofoils do not go through the Dam either. And they no longer go all the way to Chongqing, so ground transport from Chongqing to downriver towns Wanzhou, Fengjie, or Wushan is necessary....allow a partial day for that.