Getting cheap upgrades on United Airlines Flights can be challenging, as the availability of upgrades and their pricing is determined by various factors such as flight occupancy, fare class, loyalty status, and operational considerations. However, here are some strategies that may increase your chances of securing a cost-effective upgrade:

1. Join the MileagePlus Loyalty Program: Enroll in United Airlines' frequent flyer program, MileagePlus. Earn and accumulate miles by flying with United or its partner airlines, and work towards achieving elite status levels. Elite members often receive complimentary or discounted upgrades based on availability and their loyalty tier.
2. Bid for an Upgrade: United Airlines offers a program called PlusPoints, which allows eligible customers to bid for upgrades using a combination of cash and points. Keep an eye out for eligible flights and consider submitting a bid through the PlusPoints program. Successful bids can result in cheaper upgrades compared to purchasing them outright.
3. Upgrade at Check-in or Departure: Sometimes, unsold premium seats may be available at check-in or during the departure process. If you are willing to take the chance, inquire about the availability of upgrades at the airport. United Airlines may offer discounted upgrades to fill those seats, especially if the flight isn't fully booked.
4. Fly During Off-Peak Times: Upgrades are more likely to be available during off-peak travel periods when flights tend to have lower occupancy rates. Consider flying on less busy days or during non-peak travel seasons to increase your chances of securing a cheaper upgrade.
5. Use Upgrade Instruments: If you hold certain credit cards or have specific elite status with United Airlines, you may receive upgrade instruments or certificates as part of your benefits. These instruments can be redeemed for upgrades, either through online reservations or by contacting United's customer service.
6. Keep an Eye on Promotions and Special Offers: United Airlines Reservations occasionally runs promotions or special offers that provide discounted or complimentary upgrades. Stay informed by subscribing to their newsletters, following their social media accounts, or regularly checking their website for any ongoing promotions.

Remember, the availability of cheap upgrades is not guaranteed, and it heavily depends on various factors. It's advisable to research, plan ahead, and explore different options to increase your chances of securing an upgrade at a reasonable price.