Do check out Matta fair (every quarterly) and MIDE (annually) for good deals to Mabul/Sipadan.

There are many types of accommodation available if you wanna dive in Sipadan. Sipadan itself is off limits to stay there. So mostly stay nearby in Mabul, some might go for Semporna (main land), some Kapalai.

Some of the more popular places are SWV, SMART, Seaventures, Borneo Divers, Billabong, Scuba Junkie, Uncle Chang, etc.
Some of these are resorts, some are homestay, Seaventures is an oil rig, no beach, purely eat-sleep-dive, depending on what you are looking for. Also remember to check whether it includes guaranteed trip to Sipadan or not. Some places you'll need to pay extra to dive in Sipadan as they may not have enough permits for everyone.

Normally 5d4n they would give u confirm 1 day Sipadan.
Seaventures, Borneo Divers, Scuba Junkies have more permits if not mistaken.
Price wise also not that expensive.
When are you planning to go?

owh. Than just wait March for MATTA fair la. Remember to ask if the package includes Sipadan.

Peak season, long holidays, doubt they will have any rooms available in the more established resorts. Sipadan permits also will be very limited since lots of people.

Also, December/January/February or so usually raining season.

If not mistaken, room quality are different. Uncle Chang is more like a budget place while scuba junkie is like one of the biggest resort.
Be warned that SJ might be packed as its christmas holidays and SJ mainly cater to westerners. So might have traffic jam on boat and under water.

Room, Scuba Junkie, SJ has twin room that is slightly more spacious compares to Uncle Chang, UC. Mattress and washroom, SJ s superior... UC rooms r tilted above water, mostly sea view, SJ  rooms are garden view with 2 bigger units that faces the sea and seaventures.

Food, SJ serves better quality and varieties but no seafood. SJ has a proper bar area for relaxing evening, and debriefing by most DM. Live band performs here on an evening of the week. UC kitchen can cook up above average seafood if u do buy those catches by local bajau...

Diving, the SJ dshop is like 2times of UC.. Well, we do need space for our gears
Anyhow, the SJ dive is well run and organized.
SJ DM team left me with better knowledge on sea creatures, local dive sites...
SJ owns 6 Sipadan permits, 4 dives/Sipadan day
UC does prove capable of getting more permits from Sipadan permits holders though.

Please note the above are based on experiences 2-3 years ago.

If the premium Rm700 is still well within my budget, I opt for the better chance of better experience...

Yeah... Above is my yesterday experience ...