Please stick with the prescriptions you've received from your physician instead of moving to OTC products.
Each prescription is a completely different chemical, so you may respond well to one and horribly to another. Not all are the same - even generics to brands are not legally required to be direct equivalents.
Check with your insurance to see how your out of pocket expenses work. Most insurances have formularies, an approved list of drugs that they cover at various levels. Generic is always the lowest cost, then, as Susan stated, they have tiers - tier 2 is going to be less expensive than tier 3 because of negotiations between the pharmacy benefit manager (usually the insurance company) and the manufacturer.
Visit the sites for the products and verify their coupon programs. Some coupons are available every time you fill the prescription. Others have a limited offer. Some companies (pharmaceutical) create coupons that will match competitor's prices.
If your insurance is set-up that you pay a percentage of the cost of the medication, start shopping around. All pharmacies make their own prices based upon the AWP - Average Wholesale Price. It's required by law that pharmaceutical companies provide this equivalent to a car's MSRP, then pharmacies choose whether to go above or below. Grocery store pharmacies are usually least expensive. Surprisingly, Walmart, Costco, Sams Club usually fall right into the AWP cost. CVS/Walgreen's tend to be the most expensive. This doesn't apply if you pay a flay co-pay, and most pharmacies will price match their competitors if asked and provided proof of the competitor's cost.
For Nasonex, the coupon is $15 every time you fill the prescription up to $180/year https://www.nasonex.com/nasx/application?namespace=coupon...
For Singulair, the coupon is a one time $20 coupon: http://singulair.com/montelukast_sodium/singulair/consume...
You should also look into programs for Rx drug discounts through TogetherRx (http://www.togetherrxaccess.com/Tx/jsp/home.jsp) and the National Association of School Nurses (http://www.nasn.org/portals/0/membership/prescription_dru....
I'm a long time veteran in the pharmaceutical industry and would be happy to answer any other questions you have - I do not work for the companies that make either of these products (or any allergy medication, for that matter). I hope this information is helpful.