If you consider a Tamron 90mm macro expensive, then you will have to consider the cheap setups :
1. Raynox macro converter (~$120)
2. Extension tube (~$20)
3. Reversed lens ring (~$15) and a cheap 50mm prime lens that can maatch to your lens filter size.
All these can produce very good o/p and many can be more than 1:1, but they have their drawbacks in ease of use and lack of versatility.
Frankly, the Tamron is already very cheap compared to other lenses of its type.